Apart from the problems of water condensing on the walls (which you've already met), you could suffer from mildew.
Real rain comes from water vapour which rises until it cools enough by expansion, reach 100% humidity which allows the vapour to condense into microscopic droplets - which we call "clouds" if its up high, or "fog" if its down low.
The microscopic droplets then collide many times until they get big enough to form droplets, which fall to earth as rain.
I've heard that the NASA Vehicle Assembly Building is so tall that clouds can form inside it (if they didn't have air-conditioning).
But in this small room, it is going to be hard to maintain 100% humidity, and the microscopic droplets of the fog are going to hit the wall or roof before they get big enough to form a raindrop. Even with a water-repellant coating, the tiny droplets will run down the wall.
I don't think that cooling the high air will work well, because cool air sinks down to where you are trying to heat up the water.
I think your best approach would be a water hose suspended under the roof, with tiny holes in the hose. This could make a rain of droplets.
Just ensure that the walls and floor are waterproof, with a drain, and a non-slip floor - like a giant shower stall!