Moved this to the FS Upgrade forum - a lot of the creators are still active and can be found here, or at least the MediaVP people, who will know who to contact in most cases.
As for the legal ramificaions, well, it's iffy. Personally, I don't see anyone extracting [V] models from Sins, re-poffing them and then using them to repopulate FS2 or something insane like that, so it's not like directly pirating the game. Besides which, 95% of the people likely to play the mod will be those who've bought and played Fs2 anyway, and for the minority, maybe including a link to Good Old Games in the readme would be an idea?