[MrForbes]

I think that if it is insulated so it cannot come in contact with a human, it could be legal. There is no specific prohibition, and the only hazard I can think of with it is frostbite.

One that we have discussed quite a bit but never had the nerve to try, is a hydraulic system using water as the fluid. It's not hazardous, it won't damage very many things, and we've been building underwater robots for years so we're quite familiar with dealing with water and electronics and mechanicals at the same time.