They stored the stones in a suitable place on the temple mount, until
the coming of a prophet who could determine what to do with them.
This rules out the possibility that John referred, speaking of "these stones," meant the stones that are described in 1 Macc., because the Temple Mount was in Jerusalem and John was speaking on the shore of the Jordan River.
The stones are indeed metaphorical. They include non-Jews but probably also Jews who did not observe the Law. Some of the Pharisees, in fact, already affirmed that gentiles could convert to Judaism. This is evident from Jesus' statement in Matthew 23:15
“You traverse sea and land to make one convert." It is confirmed by the traditional story of Rabbi Hillel welcoming gentiles who wished to study. Sadducees, on the other hand, tended to be elitist. In my opinion they would be more likely to oppose conversion, but there is no clear biblical evidence for this.
As far as precedents in the OT, there are quite a few. Most of them are not explicitly stated but may be deduced from the context. All of the women listed in Matthew's Gospel as ancestors of Jesus (except Bathsheba and Mary) were non-Israelites. Tamar, apparently a Canaanite, married into Judah's clan. Whom Rahab of Jericho married is not clear. Some sources say Caleb, but in any case the NT is clear that she must have married an Israelite. Ruth in particular must have converted, as she was adopted under Jewish law as the wife of Boaz and became the great-grandmother of King David. Bathsheba was the wife of Uriah the Hittite. His conversion is implied by the fact he served as a commander in Israel's army and honored the laws of holy war by refusing to sleep with his wife when he returned Jerusalem before returning the battlefield. Jael the Kenite, praised as "most blessed among women" (Judges 5:24) is another example of a probable convert.
Whole tribes also may have converted: Kenites, Gibeonites. We should also consider that many other non-Israelites assimilated into Israel after the land was conquered.
The situation in the 2nd Temple period is harder to determine. Ezra and Nehemiah opposed intermarriage, apparently even with women who converted. However, Malachi may have taken issue with this policy when he prophesied:
Malachi 2:
15 Did he not make them one, with flesh and spirit?
And what does the One require? Godly offspring! You should be on guard, then, for your life,
and do not break faith with the wife of your youth. 16 For I hate divorce,
says the Lord, the God of Israel...
It seems that for Malachi, probably writing in the same time period as Ezra, intermarriage was allowed as long as the woman converted and/or agreed to raise the children as Jews.
Conclusions: John referred to stones metaphorically, certainly referring to non-Jews but possibly also to the "lost sheep of Israel" who did not observe the Law. OT precedents for conversion are fairly well attested. Many of them are from the early period of Israel's history. While the reforms of Ezra and Nehemiah made conversion more difficult, it was still practiced.
