Dude! I want pictures of this if he does it. Sounds like a really fun idea.
As for the question: I don't see why you couldn't put it in the tank with the betta as long as it's throughly cleaned and any small pieces (like those tiny little reflector pieces) secured so they don't accidentally pop off and somehow the betta manages to ingest it. I doubt the betta could open its mouth that wide, but it's better to be safe than sorry.