Unique Sides, Please

I’m making a resource pack, and I want the bottom of the Mycelium block to be different from the normal dirt texture. As far as I can figure out, this is impossible currently. I’d like it to be possible, and that’s the gist of the request, because at the moment, I’d have to finish everything else, save the texture pack, and manually redo the bottom of the mycelium block, and anything else I might want unique.