I am trying to answer your specific question:

When do we first see an explicit claim that "works of the law" in Galatians 3:10 refers to a small subset of the law (ritual, ceremonial, etc.), as opposed to the Mosaic law generally?

I believe the closest to what you are seeking is in the writings of Theodoret of Cyrus (393-458). He is not addressing his comments to the Galatians passage in particular, but is speaking on the topic of what in the Old Testament law should be studied and what should be disregarded.

Just as mothers of just-born infants give nourishment by means of the breast, and then light food, and finally, when they become children or youths, give them solid food, so also the God of all things from time to time has given men a more perfect teaching. But, despite all this, we revere also the Old Testament as a mother’s breast, only we do not take milk from there; for the perfect have no need of a mother’s milk, although they should revere her because it was from her that they received their upbringing. So we also, although we do not any longer observe circumcision, the Sabbath, the offering of sacrifices, the sprinklings — nonetheless, we take from the Old Testament a different benefit: for it, in a perfect way, instructs us in piety, in faith in God, in love for neighbor, in continence, in justice, in courage, and above all it presents for imitation the examples of the ancient saints.

Brief Exposition on the Divine Dogmas

Theodoret is saying that there are some things that are commanded in the Torah that no longer need to be practiced:

• Circumcision (e.g. Genesis 17:10-14, Leviticus 12:3)
• The Sabbath (e.g. Deuteronomy 5:15)
• Offering of sacrifices (e.g. Deuteronomy 12)
• Sprinklings (e.g Leviticus 4:7)

Among the things that he lists as things we can take from the Old Testament are love of neighbor and faith in God - both of which one could argue are commandments found in the Torah (Leviticus 19:18, Deuteronomy 6:5).

There are probably stronger texts which make my point, but I would submit that in Theodoret we find evidence of a Church Father teaching that not all of the commandments in the Law are to be dismissed.

There are many other writings of earlier Church Fathers that condemn Judaistic practices in general (e.g. Ignatius, Clement, Justin Martyr), but I believe this is the earliest work that distinguishes between what Christians should and should not take from the Old Testament Law.

The Apostolic Constitutions address this verse specifically, in Book VI. The Constitutions are believed to date from the late 4th century, but the first 6 books appear to be based on the Didascalia Apostolorum, which dates to perhaps the late 3rd century and is supposed to be a faithful handing down of Apostolic Tradition. The commentary on Galatians 3:10 is contained in Section IV.25, "How God, on account of their impiety towards Christ, made the Jews captives, and placed them under tribute":

Because, indeed, they drew servitude upon themselves voluntarily, when they said, We have no king but Cæsar [John 19:15] and, If we do not slay Christ, all men will believe in Him, and the Romans will come and will take away both our place and nation [John 11:48]. And so they prophesied unwittingly. For accordingly the nations believed on Him, and they themselves were deprived by the Romans of their power, and of their legal worship; and they have been forbidden to slay whom they please, and to sacrifice when they will. Wherefore they are accursed, as not able to perform the things they are commanded to do. For says He: Cursed be he that does not continue in all things that are written in the book of the law to do them. Now it is impossible in their dispersion, while they are among the heathen, for them to perform all things in their law. For the divine Moses forbids both to rear an altar out of Jerusalem, and to read the law out of the bounds of Judea [Deuteronomy 12]. Let us therefore follow Christ, that we may inherit His blessings. Let us walk after the law and the prophets by the Gospel. Let us eschew the worshippers of many gods, and the murderers of Christ, and the murderers of the prophets, and the wicked and atheistical heretics. Let us be obedient to Christ as to our King, as having authority to change several constitutions, and having, as a legislator, wisdom to make new constitutions in different circumstances; yet so that everywhere the laws of nature be immutably preserved.

Although not writing on Galatians 3:10 specifically, the Apostolic Father Barnabas (one of the Seventy and a companion of Paul) dedicated a large portion of his Epistle on how Christians were to regard the Jewish Law. In it he writes (2:4-6):

He hath revealed to us by all the prophets that He needs neither sacrifices, nor burnt-offerings, nor oblations, saying thus, What is the multitude of your sacrifices unto Me, saith the Lord? I am full of burnt-offerings, and desire not the fat of lambs, and the blood of bulls and goats, not when ye come to appear before Me: for who hath required these things at your hands? Tread no more My courts, not though ye bring with you fine flour. Incense is a vain abomination unto Me, and your new moons and sabbaths I cannot endure [Isaiah 1:11-14]. He has therefore abolished these things, that the new law of our Lord Jesus Christ, which is without the yoke of necessity, might have a human oblation.

Marius Victorinus

The earliest clear instance of this interpretation of this verse that I've found appears to be in Marius Victorinus's commentary on Galatians (written mid-4th century). He describes the "works which belong to Christianity" as:

those works which the apostle frequently commands (and also what has been commanded to him: let us be mindful of the poor) and the additional precepts for living which are included in this apostle's writings. Each one of these works is commanded by the apostle to be fulfilled by every Christian.

Against these moral instructions he contrasts the "works of the Law":

The works of the Law, then, are something else: religious observances, obviously, offerings of a lamb (although the Passover has now been fulfilled through Christ); and there are further works which they do as well, pertaining to circumcision and foods to be observed or prepared. (source, emphasis in original)

Sadly, a large gap appears in Victorinus's commentary immediately after this section, so we don't know how he develops the theme. A modern editor, Stephen Cooper, argues that he is pointing out "that there are other works whose obligatory performance by Christians do not bring a curse," not that justification comes from such works. Even so, the distinction is made – Victorinus makes it clear that he sees at least some aspects of the moral law as not in view in Galatians 3:10.

Origen

An earlier example of this view is found in the writings of Origen (early to middle third century). Unfortunately, his commentaries on Galatians have been lost, but it seems likely that something like the following would have been included in them. In his commentary on Romans, 8.7.6, he writes:

One should know that the works that Paul repudiates and frequently criticizes are not the works of righteousness that are commanded in the law, but those in which those who keep the law according to the flesh boast; i.e., the circumcision of the flesh, the sacrificial rituals, the observance of Sabbaths or new moon festivals. (source)

So we see that this is certainly not merely a "medieval" interpretation; at least some in the early church held to it as well.