After a brief discussion in #scp, zookeeper made this happen:
Currently produces square boring clouds. It's still N times better than not having clouds at all, but the clouds need to be about 20% cooler before this can be declared functional.
Set of problems includes:
-particles vanishing as their centre point goes off the screen
-particles rolling with camera (billboarding affects pitch/yaw/roll when it should only affect pitch and yaw)
-occasional slowdowns
-problems with controlling the size of individual particles (and thus the size of the clouds in general). Currently they are rather small.
Regardless, as this proof of concept shows, I'm confident that we'll eventually get properly working clouds in the game.
And yes, I do realize there are no clouds in the Moon, but that was the first mission I thought of when I needed a mission with terrain in it...