IIRC the Pizza map got deleted when the last big terrain change was implemented. As it used custom textures these would have to be redone by the map designer to the current terrain format and for whatever reason this was not done.
Currently the problem with a winter MA map is that there are no winter objects in the game. It would be quite possible and allowable to create an MA map with snow texture over most of the land surface, but all the buildings, trees, hedges, rocks, roads, shore batteries etc. would have no snow covering them and would stick out like sore thumbs against the white ground. MA map designers are not allowed by HTC to alter the textures on these objects, partly because all those textures would make the terrain size huge and partly because errors made in altering these objects could crash the game. Maybe the new terrain system will make this possible in some way, but I haven't seen anything yet that would support that.
I did consider having some places on my MA map where a few high SPs were above the snow line so players could fight in a winter terrain and use winter GV skins for camo. Then I realised players would just turn off skins to make their opponents' tanks stand out against the white and gave up on the idea.
