What a nice idea! 
I think we should give modifed nebula poofs a try. Modify only one of them as set it as the only poof in FRED, that'd create an interesting effect.
While modifying motion debris may seem the most effective solution I'd like to see what a "particular" nebula can do. We can even use thunders with no textures and change thunder sounds to get a proper noise ingame! 
I know, there's no rain in space, but think about atmospheric missions for a second... 
If the nebula poof alignment can be fixed, then it can be used to create a very good effect of "background rain" and to limit the visibility, but the close proximity rain/snow would probably be better with motion debris system.
What we need is two features (although either of them would be great) from the coders:
1. A way to adjust motion debris density and consistency so that it uses particles rather than models, and can be set to drift to a direction with some velocity
2. A way to fix the nebula poofs to a single gameworld vector.
Clouds are going to be a problem, though.