I've always wanted this.
Especially on stuff like the slopes.
You would have to make models for all of the caves, because terrain doesn't work like that, so you would have to include them in the map zip. Then, you would have to make sure that they used the snow textures or whatever you wanted for it. Your model would probably look unrealistic when sitting all alone, but then you would probably be able to move the terrain around it so that it fit in.
Unfortunately, this would mean that tunnels going straight through the mountain are right out, unless you open up the terrain file somehow and delete certain parts of it, which could take ages to do, and it probably wouldn't work.
I believe that the way that terrain is saved uses vertexes to figure out exactly where the terrain is.
I gtg, maybe talk about this more later.