Might help to mark 'detail' for the shader stage of the rain texture so it could be skipped for r_detailtextures 0.
Heck, even doing it through cgame's local entity system as a mod (where the smoke trails happen - this is how Annihilation implements it) would be bad since it would hit the small limits of them eliminating rocket trails, weaponhits and rail trails.
I am not a mapper. I am just spewing technical artist theories here.
Edited 10 minutes after the original posting.
In a map I have fog but it is glitched, there are no leaks or errors that I've made which I'm aware of and there is only one visible surface to the fog. The fog has an odd "solid" look on the surface (the look you get when there's more than one visible surface), the only thing that I can think of that may have made it this way is the curves that I placed on the walls where the fog is (which have been caulked from behind). Do curves have an affect on fog?
I'm not at my main PC at the moment, so I can't tell you the names of the one you are after.
Now here's how it should look: imageshack.us/a/img96/2655/xr5m.jpg
It has a smoother transition from the fog instead of having an "edge" look to it.
Edited 6.93 minutes after the original posting.
Edited 8.48 minutes after the original posting.
I just compiled it without lighting because it takes a while to do it.