I had a look at doing something like this when AH3 was in beta, but unfortunately it won't really work for an MA terrain. The default summer terrain tile set has a couple of winter tiles in it but they are really only intended for capping tall mountains. As I recall (this was a few years ago) there are several problems:-
The clutter in the winter tiles (trees, grass etc.) is not snow covered and so looks strange up close, this isn't too important for mountain peaks but would look odd in an area where ground fighting takes place. You aren't allowed to replace this in an MA terrain.
There is no way of making the various bases and strats match the all of the terrain so the bases look like dark square holes in the white terrain. Even if you were allowed to alter all the base textures to have snow covered buildings and terrain (which you aren't on an MA terrain) this would then apply to the bases in the summer areas too.
The tiles in AH are designed to "stack" in a specific order, with a beach sand texture at sea level at one end and the snow textures on the mountain peaks at the other. Putting a snow texture in the middle of the stack creates an odd-looking border around the snow area comprised of all of the in-between tiles' border textures.
BTW when the AH3 winter tile set was introduced I considered using it to make an entirely winter MA terrain. I figured it would be something a bit different and it would get my winter skins some MA use. However when I thought about it a bit more I decided against doing it. The problem is there are plenty of players who will go to any length to gain an advantage in the game. Remember in AH2 when GVers would turn off every type of ground clutter making the game look like crap just so they could see enemy tanks easier? In a winter terrain what would happen is many players would pick a winter skin for their own tank and then click the "turn off other players' skins" button so the opposition tanks displayed only the default skin. As a skinner I don't want to encourage players to use that button as them might not turn other players skins back on later. I believe replacing the default skins with winter ones is also forbidden in an MA terrain.
