Is the Intermingling of the Lost Tribes a New Idea? [QUESTION FOR DAN]

Dan, I'm going to offer 2 quotes from Joseph Klausner and then provide the source text from Syriac Baruch and IV Ezra.  I'm just curious:  what are your thoughts about this ancient Jewish belief that the lost tribes intermingled with Gentiles so that their original identity was completely lost?

Here are the excerpts with source text:

"Resurrection will be attained both by those who were originally righteous but later 'mingled themselves with the seed of mingled peoples,' and by those who were originally wicked but later joined with the people of Israel, that is to say, the Ten Tribes and the righteous proselytes (42:1-5)," Klausner, The Messianic Idea in Israel, pg. 347

"IV Ezra emphasizes with all force the expectation of the attack of the nations (the war of Gog and Magog) in the Days of the Messiah, and in connexion with this the ingathering of the exiles and the return of the Ten Tribes," ibid pg. 363

2 Baruch:

41 1 And I answered and said: 'For whom and for how many shall these things be? or who will be worthy to live at that time? 2 For I will speak before you everything that I think, and I will ask of you regarding those things which I meditate. 3 For lo! I see many of Your people who have withdrawn from Your covenant, and cast from them the yoke of Your law. 4 But others again I have seen who have forsaken their vanity, and fled for refuge beneath Your wings. 5 What therefore will be to them? or how will the last time receive them? 6 Or perhaps the time of these will assuredly be weighed, and as the beam inclines will they be judged accordingly?'

42 1 And He answered and said unto me: 'These things also will I show unto you. 2 As for what you did say—"To whom will these things be, and how many (will they be)? "—to those who have believed there shall be the good which was spoken of aforetime, and to those who despise there shall be the contrary of these things. 3 And as for what you did say regarding those who have drawn near and those who have withdrawn this in the word. 4 As for those who were before subject, and afterwards withdrew and mingled themselves with the seed of mingled peoples, the time of these was the former, and was accounted as something exalted. 5 And as for those who before knew not but afterwards knew life, and mingled (only) with the seed of the people which had separated itself, the time of these (is) the latter, and is accounted as something exalted. 6 And time shall succeed to time and season to season, and one shall receive from another, and then with a view to the consummation shall everything be compared according to the measure of the times and the hours of the seasons. 7 For corruption shall take those that belong to it, and life those that belong to it. 8 And the dust shall be called, and there shall be said to it: "Give back that which is not yours, and raise up all that you have kept until its time".'

IV Ezra Chapter 13:

33: And when all the people hear his voice, every man shall in their own land leave the battle they have one against another. 

34: And an innumerable multitude shall be gathered together, as thou sawest them, willing to come, and to overcome him by fighting. 

35: But he shall stand upon the top of the mount Sion. 

36: And Sion shall come, and shall be shewed to all men, being prepared and builded, like as thou sawest the hill graven without hands. 

37: And this my Son shall rebuke the wicked inventions of those nations, which for their wicked life are fallen into the tempest; 

38: And shall lay before them their evil thoughts, and the torments wherewith they shall begin to be tormented, which are like unto a flame: and he shall destroy them without labour by the law which is like unto me. 

39: And whereas thou sawest that he gathered another peaceable multitude unto him; 

40: Those are the ten tribes, which were carried away prisoners out of their own land in the time of Osea the king, whom Salmanasar the king of Assyria led away captive, and he carried them over the waters, and so came they into another land. 

41: But they took this counsel among themselves, that they would leave the multitude of the heathen, and go forth into a further country, where never mankind dwelt, 

42: That they might there keep their statutes, which they never kept in their own land. 

43: And they entered into Euphrates by the narrow places of the river. 

44: For the most High then shewed signs for them, and held still the flood, till they were passed over. 

45: For through that country there was a great way to go, namely, of a year and a half: and the same region is called Arsareth. 

46: Then dwelt they there until the latter time; and now when they shall begin to come, 

47: The Highest shall stay the springs of the stream again, that they may go through: therefore sawest thou the multitude with peace. 

48: But those that be left behind of thy people are they that are found within my borders.