A mule in the neck will work fine with a MM in the bridge. The mule will be nice and fat and have a different texture to the MM but will not seem lacking in output
One combo I've been thinking about was a Mule neck and Warpig bridge (I'm more into the Alnico feel than the ceramic feel). Warpig is a bit hotter than than the MM, though, I believe, so perhaps it might start to overbalance the relative outputs ...?