This would make an extremely dangerous precedent and would undo any good the Rome Treaty unsigning accomplished. While I agree that it would not be inappropriate for those individuals to stand trial for war crimes, it ought to be before an impartial American tribunal.
I can't picture any precedent any worse than taking a piece of territory a country (Serbia) had owned for over 1000 years and simply handing it over to a bunch of other people and claiming that breeding power means ownership from now on. The UN could just as easily demand that we give California over to Mexico using the same formula. In fact, we probably ought to expect it.