I remember Max Maven performing a trick where several spectators have a wrong key, 1 has the right one. Upon feeling an inclination to open the lock on e spectator gets up an does it. There a few marketed effects which achieve similiar things.
I had the weird idea of trying to use a combination lock. Have 3 people pick #s and have it be the correct combination. Of course getting them to pick the right #s isnt easy. Well I haven't thought of a method yet. any ideas?