Attach new interior wall to ceiling drywall?
The correct way to do it is to remove some of the drywall from the ceiling, and install cross braces between the joists.  You'll then attach the top of the wall to the cross braces.  Anything less and you'll have movement in the wall, and you don't want the wall moving (especially if it's tiled).

Sounds like you've got a nice project going. Don't half-ass it, or you'll regret it later when you're redoing it.