You might also do two passes -- one of the zeppelin, the other of the fog. Then you can composite them together in AE.... The fog could be rendered against black and composited with the lighten or screen transfer mode if the alpha doesn't work. As for the billowing effect, in RDS there's an animatable feature in fog objects for swirl, chaos, etc. I'd use these sliders (start at one value, end at another value) to see what kind of effects you can pull off.... And do low res test renders to save you the pain of rendering all weekend to find out something didn't work!