Well it has no particular artistic merit but I thought I'd give it a go to see if it was possible. Seems like it could work with a LOT of further tweaking. Its basically a stack of 1000m thick cumulus cloud layers each 500m above the previous one. I've only used the one mask, obviously using more would increase the variation. A similar technique may also be useful for everything from thunderheads to campfires :) On the down side this took 29 hours to render and I shudder to think what a more complex node structure would take. Attached is the file for playing with, have fun.
An interesting experiment. :)