>>287225870
I feel like you can get a 3d camera pretty compact. Not like you need a real high def view of the face to do this sort of thing anyway. 
Otherwise yeah not even going so far as BCI. companies like emotiv were able to do quite a bit with simplified EEG. Couple of probes around the headset to measure brainwaves or muscle activity and a ml algorithm should be able to parse out expressions I'd think.