Anyone else think the costume would be about 17% more unnerving if the demon's face didn't look as if it had just been smacked with a cartoon frying pan?
But in all seriousness, I'll bet it would make a lot less difference in the dark.
I guess it would be tricky to fit an optical sensor under that under the mask, but maybe you can track eyeball motion by tracking the muscle movement around the eye?
Some quick tests (i.e. placing my hand on my face and moving my eyes around wildly) suggests that it might be doable. :) Possibly just by tracking how your nose twitches...
But eyes that are randomly wandering around are actually kinda creepy, too... :-)
With that in mind, it's almost 4.2% more unnerving than the alternatives.