Asking if “this [hallal] meat is being offered up to idols” seems to me a similar question as asking whether islam is idolatry (from a Catholic point of view).

What is idolatry?

A longer discussion of this definition can be found in the Dictionnaire de Théologie Catholique (“IDÔLATRIE”). The Church Fathers understand idolatry as a worship rendered to false gods (Clement of Alexandry, St Gregory of Nazianzus). Origen is more precise and distinguishes the image (είκων) – true representation of an existing thing – from the idol (εἴδωλον) – false representation of a thing that doesn’t exist.

This sense is also the one of the Holy Scriptures (if you need more details about it please see the little bibliography at the end of my answer). That’s why idolatry must be defined as (translation of the formulation by F. Prat, Dictionnaire de la Bible, at “IDÔLE”):

The supreme and absolute cult rendered to any other than the unique and true God.

So the question is: (according to Catholic Church) do they render a cult to the true God?

Is the Allah of Islam the same as the God of Christians?

According to Athanase Creed (recited by Catholics at Prime on Sunday, and admitted by Protestants), “the Father is God, the Son is God, and the Holy Spirit is God, and yet there are not three Gods but one God” (Ita Deus Pater, Deus Filius, Deus [et] Spiritus Sanctus. Et tamen non tres dii, sed unus est Deus).

All human have the same God, in the sense that He is our Creator, whereas it doesn’t mean that all humans believe in Him: any God that isn’t triune is not the true God of Christians.

Even if they are right to be monotheistic, Muslims believe (Shahada^W):

There is no god but God. Muhammad is the messenger of God.

And in fact Muslims oppose with force and sometimes violence against Christians precisely because of the trinitarian dogma. That’s why it remains clear that, from a Catholic point of view, they worship an idol that they name Allah and that isn’t the triune God.

Hallal meat

So now is hallal meat being offered up to Allah? It seems so: according to Wikipedia:

Dhabīḥah (or zabiha, Arabic: ذَبِيْحَة‎‎ dhabīḥah IPA: [ðæˈbiːħɐ], 'slaughter'(noun)) is, in Islamic law, the prescribed method of ritual slaughter of all lawful halal animals (goats, sheep, cows, chicken) excluding locusts, fish, and most sea-life. Unlawful animals like pigs, dogs, lions, bears, etc. are not allowed to be slaughtered or zabihah. This method of slaughtering lawful animals has several conditions to be fulfilled. The butcher must be Muslim, the name of God or "In the name of God" (Bismillah) must be called by the butcher upon slaughter of each halal animal separately, and it should consist of a swift, deep incision with a very sharp knife on the throat, cutting the wind pipe, jugular veins and carotid arteries of both sides but leaving the spinal cord intact.

Dhabihah

What we have demonstrated is that, according to a catholic definition of idolatry and to the fundamental differences between Islamic Allah and catholic dogmas about what is God, Allah is in fact an idol from a catholic point of view.

Furthermore, it seems that dhabīḥah is a sacrifice to this Allah. So we may say: “Yes, hallal meat is being offered up to idol.”

From a Catholic point of view, it is meat sacrificed to idols, and so 1 Cor 10, 2 Cor 6, Acts 15:28–29… could be applied to hallal meat. By the way, this answer isn’t exactly an answer of the Catholic Church as it seems that no text of the Magisterium specifically deals with halal meat (and I personally find it logical): this answer is only an argumentation build on what the Catholic Church teaches.

As a conclusion, this interesting commentary of Blessed Innocent V on 1 Cor 10:28–32 (usually included in Saint Thomas Aquinas commentaries, translation by the Aquinas Institute):

If I partake with thanksgiving, why am I spoken ill of for that for which I give thanks? [n. 568]

Above he warned them to beware eating the food of idols, and he has given four reasons for his admonition: here he teaches, third, the way to beware the abovementioned things, by showing how one is allowed to eat, and how not.

First therefore he shows whether it is licit to eat;

second, when it is not licit, at but if any man says;

third, what to attend to in either case at therefore, whether you eat (1 Cor 10:31).

read more…

Bibliography

On the definition of idolatry

• Dictionnaire de Théologie Catholique, “IDÔLATRIE”

On the sense of idolatry in the Holy Scriptures

• Dictionnaire de Théologie Catholique, “IDÔLATRIE”
• F. Prat, Dictionnaire de la Bible, “IDÔLE”
• Hagen, Lexicon biblicum, the definition of the word “[IDOLOLATRIA][2]”

Commentaries on Aquinas

• Translations by the Aquinas Institute are available online

In addition: some exact references to the teaching of Catholic Church

Believing in God cannot be separated from believing in the One he sent, his ‘beloved Son’

For a Christian, believing in God cannot be separated from believing in the One he sent, his ‘beloved Son’, in whom the Father is ‘well pleased’; God tells us to listen to him. The Lord himself said to his disciples: ‘Believe in God, believe also in me’ (Jn 14:1). We can believe in Jesus Christ because he is himself God, the Word made flesh: ‘No one has ever seen God; the only Son, who is in the bosom of the Father, he has made him known’ (Jn 1:18). Because he ‘has seen the Father’, Jesus Christ is the only one who knows him and can reveal him (Jn 6:46; cf. Mt 11:27). (Catechism of the Catholic Church, no. 151)

Islam denies that Christ is truly the Son of God

He [Mohammed] says that the Christ is the Word of God and His Spirit, but a creature and a servant, and that He was begotten, without seed, of Mary the sister of Moses and Aaron. For, he says, the Word and God and the Spirit entered into Mary and she brought forth Jesus, who was a prophet and servant of God. (St John Damascene, Concerning Heresies, 101: PG 94, 766)

First of all we must observe that Muslims are silly in ridiculing us for holding that Christ is the Son of the living God, as if God had a wife. Since they are carnal, they can think only of what is flesh and blood. For any wise man can observe that the mode of generation is not the same for everything, but generation applies to each thing according to the special manner of its nature. […] So generation should be understood of God as it applies to an intellectual nature. (St Thomas Aquinas, Compendium on Reasons for the Faith against Muslim Objections, 3)

O People of the Book, exceed not the limits in your religion, and say not of Allah anything but the truth. Verily, the Messiah, Jesus, son of Mary, was only a Messenger of Allah and a fulfilment of His word which He sent down to Mary, and a mercy from Him. So believe in Allah and His Messengers, and say not ‘They are three.’ Desist, it will be better for you. Verily, Allah is the only One God. Far is it from His Holiness that He should have a son. To Him belongs whatever is in the heavens and whatever is in the earth. And sufficient is Allah as a Guardian. (Koran 4:172)

I think it pretty logical the answer is NO. Christians must not eat halal. Only a muslim can bless the meat as he slaughters it. This blessing is to God, but which God? Since a Muslim's god is allah, then the blessing is to allah. Allah according to Catholic Church Magisterium is a pagan god, diabolical, and therefore Islam is a demonic religion.

It would be sacrilege and a breaking of the 1st commandment to eat halah by any Christian.

Short Answer: No.

From the proceedings of the Ecumenical Council of Florence.

"[The holy Roman Church] firmly believes, professes and teaches that every creature of God is good and nothing is to be rejected if it is received with thanksgiving, because according to the word of the Lord not what goes into the mouth defiles a person, and because the difference in the Mosaic law between clean and unclean foods belongs to ceremonial practices, which have passed away and lost their efficacy with the coming of the gospel. {snip} "It condemns, then, no kind of food that human society accepts and nobody at all neither man nor woman, should make a distinction between animals, no matter how they died; although for the health of the body, for the practice of virtue or for the sake of regular and ecclesiastical discipline many things that are not proscribed can and should be omitted, as the apostle says all things are lawful, but not all are helpful." ~(Ecumenical Council of Florence, Session 11, 4 February 1442)

As I have not found this to have been overturned in a later council, this sensible position would remain current unless a papal encyclical or bull offered further guidance.

Frame Challenge

The imbedded assertion in your question, that halal or kosher butchery is a religious ceremony of worship to an idol is unsupported. A modest Google search found a variety of Christian content web sites, and posted articles by various pastors -- almost all Protestant or Evangelical -- that raise this question. While I'd offer that your question is asking about the wrong denomination, I went to Catholic answers dot com and saw a similar concern being discussed.

Ora et labora

As to praying while doing one's work, be ye butcher or baker, bus driver or candlestick maker, consider:

• 1 Thessalonians 5:17 and the Apostle Paul's exhortation to pray continually

• The Benedictine principle of "ora et labora" (Work and prayer) which many Catholics interpret as unifying work and prayer; in this sense, all that we do is an offering to God. (That last point has become a common teaching at our parish in the past 10 years, and is usually matched with the seasonal topic of stewardship).

  One cannot assume, from a Catholic point of view, that just because someone may be praying while engaging in one's vocation that a religious rite, or the worship of an idol, is underway.

  But as Catholics, we believe that this work is more than earning a living; it is a prayer of our love for the people we support, as well as a message to God. Work shows God how much we love him, and it displays his love to those around us. In his epistle, St. James says, "Show me your faith without your works, and I will show you my faith by my works" — that is to say, we cannot separate our faith lives from our working hours. Faith is not meant to be confined within the walls of a church, only brought out on Sundays and holidays; we are meant to live our faith, communicating it through the world we are building, behind desks, in classrooms, offices, cars, kitchens, and homes. No job is too big or too small, and all have equal dignity when they are performed for love of Christ.

The Roman Catholic Church does not teach that food certified by either the appropriate Jewish authorities as meeting requirements for Kosher certification, or by Islamic authorities as meeting requirements for Halal certification are foods offered to idols. The Catholic position is that the God of the Jews and the God of Muslims is the same God as the Christian God,, and therefore not an idol.

In explaining different understandings between most Jews and most Muslims, from the understanding that most Christians have of God, Catholics teach that Jews and Muslims, and frankly many non-Catholic Christians, too, have (at best) an incomplete knowledge of the nature of God.