I figured out a way to make mia_material_x and mia_material_x_passes behave like the other materials when dragged and dropped. It involves a modification to a script called connectNodeToAttrOverride.mel which contains the following encouraging suggestion
//This procedure is provided as a hook for customers to allow
//you to redefine the behaviour of drag and drop.
So I added a few lines :)
Just drop the mel in your local scripts folder and restart maya.