Saying that, I wonder what the minimum number of displacement maps would be acceptable. For example, frowning for forehead wrinkles, a smiling map for cheek/nose/eye creases/wrinkles etc. They could maybe be linked to the appropriate morphs using dependent parameters possibly, if DPs can be used to animate materials.
Hmmm, intriguing thought. And it certainly wouldn't real-time! :) Could be worth it for animations though.