My reading of the rule stands with many others. If you want to levitate whiffle balls inside the volume of your robot, go for it. You may not, however levitate whiffle balls outside of your volume. I think air cannons will be legal.
Enjoy.
https://www.youtube.com/watch?v=9QBk1c5hfhE
I believe the cuts on the ball in the video are different than the FRC game pieces, but somebody should give this a shot - just for fun... or to win.