Has anyone ever attempted such a thing before?
No experiences here or with development on the xbox, just a possible fallback idea, if you can't share game data between games:
Can't you check certain archievements (don't know if this is possible) ? This way you can give additional options to players who have played through your first game by checking for certain archievements.
Edit: it seems that save game sharing should be possible (Mass Effect, Dragon Age).