Well, I never did find the feature I was searching for in Maya. But I've found work-arounds for both problems.

To prevent an extra material from being exported (the one assigned to the collision model), just assign one of the materials used by the static mesh to the collision model as well (duh!). So simple it hurts.

To order the material list exported by the ActorX plugin - it turns out (MUCH to my surprise) that the .ase file is in ASCII format, and what's more it looks like a programming or page layout language. Since the necesity to order materials is a rare occurance, it was easy enough to just cut/paste in the .ase file before importing into Unreal. Also so simple it hurts.

A classic case of not being able to see the forest for the trees! :blush: