Quote - One of my first plays with animating displacement here, using a movie as displacement map

http://www.youtube.com/profile?user=VJflickeringlight#p/u/85/-Lby7pMbNhg

Its not pretty, and is a subject I've returned to a couple of times, but with limited success

http://www.renderosity.com/mod/gallery/index.php?image_id=1391617&user_id=11357&np&np

is a still from another animated displacement, in this case the map was a bitmap and I animated the scale of displacement. this method would probably be more suitable for your suggestion of animating expressions as a bitmap can have a much higher res than most movies and subtle gradations of the displacement amount I can see working quite well for facial creases etc.

as for loading lots of maps, might it be possible to use animated blender nodes to "edit" the appearance of different parts of the map instead?

Did you ever try turning down the displacement values? That would be a nifty Monster transformation clip but if you tried lower displacement values you might get something human from that method.