how do you do a roll in the air without something to bounce off of?
The first part is true, the second is not. Open rolls use double strokes, where you hit the drum twice with one wrist motion. That's not possible if the stick isn't rebounding.
Playing single strokes at high speed wouldn't be possible here either. You're doing twice as much work if you have to pull the stick back up after each stroke.
It's not really a common thing to practice, and I only speak from my own experience as a drummer. But after a short while it's trivial to simulate an open roll fairly simply.
Anything high-performance is definitely out, though.