Hate to revive this, but...
So using these methods (cyphr's has been the successful one for me so far) how would one include slope and altitude masking, since the final blend shader is being inverted? I didn't see anything mentioned on it in the thread so it has to be something really simple, but I can't for the life of me figure it out.
I've tried everything I could think of - inverting the surface layer attached to the stones mask, but to no avail.