there is a natural body function that i think could be exploited for a wetting file that i don't think anyone has touched on yet. many people experience the need to pee just an extra dribble after they are done peeing. as i understand it this may be caused by one kidney emptying just a little slower than the other.
i think hypnosis could exploit this phenomenon to encourage wetting outside of the toilet. perhaps the suggestion could be something along the lines of suggesting the listener will subconsciously hold back a bit of pee and be unable to release it until he gets off the toilet or steps away from the urinal, at which point it will release automatically.