I think it would be the same as trading between hg/ss and d/p/pt. The time difference wasn't brought up there, so why should it be now? Though I agree that there will probably be an event to unlock the full trading capacity between the games, whether it be beating the elite four or another side story.
As for Kyurem's forms, the technology in pokemon has gotten more advanced since d/p/pt. I can see them releasing a patch through dream world, hooking up to the new game, or even wifi to add Kyurem's new forms to the B/W pokedex. How hard could it be, really? (Remember when hooking up FR/LG to RSE unlocked the National Pokedex?)