Y_M, try setting up a scene in whatever 3D package you use with your rock model and a single directional light above and then bake the lighting into a texture. You can then use it as a base for a snow mask. It'll make it easy to have the snow on just the top parts of the rock.
Wow, Awesome Ferg. ME! For some reason I thought the rocks were gemstones type rocks or something. I know you said you're done with it, but maybe in the future, try some shaders that will make each color reflect like gemstones? cuz I think that may look AWESOME!!!