Thanks everyone. At Ken's suggestion I tried using specularity but that didn't do anything with flat layers.
I think the best way to do it is to put the face in front of the helmet. Using Choreography as a multi-plane camera has a lot of exciting possibilities and I'm looking forward to working with this. All you've got to do is scan things in to photoshop, use rotoscope to make a grid that's the same shape, bone it up and shazam! you're good to go. But, you know, it would be nice to be able to prevent the color of a front layer from affecting layers that are behind it.