OK, I wasn't entirely serious but now I'm starting to think it might work. Epoxy rather than superglue would give you a better shot at getting the placement right. There's a bunch of places you could keep the third magnet.
If you want to go the musical route, no need for a string. Add a reed and you can play the seat tube / seatpost a bit like a trombone. If you have perfect pitch you're done.