Sorry, this won't work. WMS maps need external utility for lat-lon to tile addressing translation (/usr/bin/cs2cs IIRC). Even if this utility is installed, layer's tile addressing scheme must match to underlying map's. In other words, layer's tiles must exactly match tiles they are drawn on. All maps repositories supported by MM by default, have google scheme (which is WGS on top of sphere).

Layers can be implemented to support any tile addressing, but this would be extremely inefficient and too complicated (we must deal not with whole tile, but with part of it).

I did a quick search for weather services for google maps, and found nothing usefull. All services display weather information as points on top of google map which handled by javascript.