Seattle, WA
I've been kicking around an idea for a San Francisco-themed pinball machine for a few years, and we really, really want to have a mode where the fog rolls in and you can't see your ball. Unfortunately even these water-based solutions still have some type of oil in them that would coat everything if it were used inside of a machine, so I think it's a non-starter for use under the glass.
FWIW we also looked at dry ice, but that the "fog" it produces in hot water is water vapor which also seems like would not be a good thing to introduce under the glass of a pinball machine.
But man... how cool would that be though? Imagine seeing all the air swirl movements and stuff from the ball moving around. 
