Are there any accounts of former Pagans being supernaturally reached by God (or an angel) with the Gospel before any human missionary arrived?

One of the first recorded examples of this is when the angels went to Sodom to warn Lot and his family to flee the wrath that was about to fall on Sodom and Gomorrah.

Gen 19 12The two men said to Lot, “Do you have anyone else here—sons-in-law, sons or daughters, or anyone else in the city who belongs to you? Get them out of here, 13because we are going to destroy this place. The outcry to the Lord against its people is so great that he has sent us to destroy it.” 14So Lot went out and spoke to his sons-in-law, who were pledged to marry a his daughters. He said, “Hurry and get out of this place, because the Lord is about to destroy the city!” But his sons-in-law thought he was joking. NIV

Any pagans or unbelievers who would have listened to the angels’ warning to Lot would have been saved.

The second example I would cite occurred when the angels announced Jesus’ birth to the shepherds in the fields. It is most likely they were Jews but there may have been pagans among them, hired to help them with the flocks. They would have watched the flocks as the shepherds went into Bethlehem.

15When the angels had left them and gone into heaven, the shepherds said to one another, “Let’s go to Bethlehem and see this thing that has happened, which the Lord has told us about.”

The third example is the clearest mentioned in the Bible, though it has not yet occurred. God's sends angels to make sure his message is clearly understood everywhere in the world because the consequences of ignoring it are so severe.

6Then I saw another angel flying in midair, and he had the eternal gospel to proclaim to those who live on the earth—to every nation, tribe, language and people. 7He said in a loud voice, “Fear God and give him glory, because the hour of his judgment has come. Worship him who made the heavens, the earth, the sea and the springs of water.” 8A second angel followed and said, “ ‘Fallen! Fallen is Babylon the Great,’ a which made all the nations drink the maddening wine of her adulteries.” 9A third angel followed them and said in a loud voice: “If anyone worships the beast and its image and receives its mark on their forehead or on their hand, 10they, too, will drink the wine of God’s fury, which has been poured full strength into the cup of his wrath. They will be tormented with burning sulfur in the presence of the holy angels and of the Lamb. 11And the smoke of their torment will rise for ever and ever. There will be no rest day or night for those who worship the beast and its image, or for anyone who receives the mark of its name.” 12This calls for patient endurance on the part of the people of God who keep his commands and remain faithful to Jesus.

This will be one of the most spectacular events the world has ever experienced. It will be the fulfillment of Jesus’ words in Mt. 24:14.

And this gospel of the kingdom will be preached in the whole world as a testimony to all nations, and then the end will come.

Yes. Read the works of Don Richardson, such as Peace Child and Eternitry in their Hearts.

From memory, one of his accounts (or I could be confusing it with another author who dealt with the same subject) relates to a people in the area of Burma. The village leader had a dream about a man with a book who would tell them things about God that they needed to know. He was instructed to choose a cow and send people from the village to follow that cow and it would lead them to the person God had chosen to speak to them. The next day he told the people in the village of his dream. He walked up to a cow, placed his hands on it and prayed. Then two men chosen from the village followed the cow. They expected it to wander a little as cows do but not really lead them anywhere distant. Instead, the cow marched them 200 miles through the jungle, stopping only to eat, drink and rest. It crossed the border into Northern India and walked up to a hole in the ground and stopped. The men went to the hole, looked down and called out. At the bottom of the hole was a Christian missionary who was digging a well. They brought him and his Bible back to their village and he shared the Gospel with them, whioch they accepted.

Are there any accounts of former Pagans being supernaturally reached by God (or an angel) with the Gospel before any human missionary arrived?

This vary thing happened in the Book of Acts with the story of Cornelius the Centurion.

In the Book of Acts, Cornelius’ conversion happens because both Cornelius and Simon Peter receive visions from God. For Cornelius, God calls him and his Gentile household to follow the crucified and risen Christ. At the time, the fact that Cornelius was a Gentile (not Jewish) was a big deal. Being a Gentile was enough to prevent him from being accepted by the nascent Jesus Movement.

Cornelius was a centurion in the Cohors II Italica Civium Romanorum, mentioned as Cohors Italica in the Vulgate. He was stationed in Caesarea, the capital of Roman Iudaea province. He is depicted in the New Testament as a God-fearing man who always prayed and was full of good works and deeds of alms. Cornelius receives a vision in which an angel of God tells him that his prayers have been heard; he understands that he has been chosen for a higher alternative. The angel then instructs Cornelius to send the men of his household to Joppa, where they will find Simon Peter, who is residing with a tanner by the name of Simon (Acts 10:5ff).

The conversion of Cornelius comes after a separate vision given to Simon Peter himself (Acts 10:10–16). In the vision, Simon Peter sees all manner of beasts and fowl being lowered from Heaven in a sheet. A voice commands Simon Peter to eat. When he objects to eating those animals that are unclean according to Mosaic Law, the voice tells him not to call unclean that which God has cleansed.

When Cornelius' men arrive, Simon Peter understands that through this vision the Lord commanded the Apostle to preach the Word of God to the Gentiles. Peter accompanies Cornelius' men back to Caesarea. When Cornelius meets Simon Peter, he falls at Peter's feet. Simon Peter raises the centurion and the two men share their visions. Simon Peter tells of Jesus' ministry and the Resurrection; the Holy Spirit descends on everyone at the gathering. The Jews among the group are amazed that Cornelius and other uncircumcised should begin speaking in tongues, praising God. Thereupon Simon Peter commands that Cornelius and his followers, "kinsmen and near friends", be baptized. The controversial aspect of Gentile conversion is taken up later at the Council of Jerusalem (Acts 15). -Cornelius the Centurion

Some would consider Mary of Jesus of Ágreda of fitting the bill, while others may not. In any case I will post it as an answer.

Mystical bilocation and effect on missionaries

Between 1620 and 1623, Mary of Jesus reported that she was often "transported by the aid of the angels" to settlements of a people called Jumanos. The Jumano Indians of New Spain (what is today Texas and New Mexico) had long been requesting missionaries, possibly hoping for protection from the Apaches. Eventually a mission led by the Franciscan Friar Juan de Salas visited them in 1629.

The abbess reported further but less frequent visits afterwards, all while she physically remained in the monastery at Ágreda. They thus are considered bilocations, an event where a person is, or seems to be, in two places at the same time. Before sending the friars, Father Alonzo de Benavides, Custodian of New Mexico, asked the natives why they were so eager to be baptized. They said they had been visited by a Lady in Blue who had told them to ask the fathers for help, pointing to a painting of a nun in a blue habit and saying she was dressed like that but was a beautiful young girl. The Jumanos visiting Isleta indicated that the Lady in Blue had visited them in the area now known as the Salinas National Monument, south of modern-day Mountainair, New Mexico, about 65 miles (104.6 km) south of Albuquerque. At the same time, Fray Esteban de Perea brought Benavides an inquiry from Sor María's confessor in Spain asking whether there was any evidence that she had visited the Jumanos.

As reports of Mary's mystical excursions to the New World proliferated, the Inquisition took notice, although she was not proceeded against with severity, perhaps because of her long written relationship with the Spanish king.

Accounts of Mary's mystical apparitions in the American Southwest, as well as inspiring passages in Mystical City of God, so stirred 17th and 18th century missionaries that they credited her in their own life's work, making her an integral part of the colonial history of the United States. - Mary of Jesus of Ágreda

The following articles may be of interest in this subject matter:

• The Angels and Their Mission
• Eternity in Their Hearts: Startling Evidence of Belief in the One True God in Hundreds of Cultures Throughout the World by Don Richardson. Fascinating accounts of how God enabled the people of different pagan cultures to understand the meaning of the Gospel.

During my first trip to Sierra Leone (2005) our team went out into the bush to a small Muslim village along a riverbank. Upon arriving our leader met with the village headman to explain our desire to declare the gospel and to ask his permission to do so in his village. The headman was around 50 yrs old (pretty old for those parts) and had been told in a dream that he would not die until someone came to him with truth from God.

We shared the gospel with every person for a week, held a worship service on Sunday, and baptized many Muslims in the Name. Within a year there was a Church established as well as a school/orphanage. Within that same year the headman passed into the presence of his Lord.

I cannot help but to believe that there are thousands of stories like this because God is neither slack nor inefficient in any regard. He doeth all things well.