I've filled whole head masks with Great Stuff numbers of times, but the faces are static (even if the prop isn't). I developed a method that works well for me --Simple, no mixing, no brushing, costs <2 bucks to do a mask.
I tape over any holes and fill any protruding features first. Once they set, I give the mask a little spray of water and do a shallow coating of the entire inside and let that set. If I want the head just as-is, I water-spray again, put a partially-inflated balloon in, and fill around it with the spray foam. The balloon will offer less resistance to expansion than will the already-set layers of foam, so there's no distortion. It stays light-weight and you can do several masks with a single can of foam. If I want to adapt it later to a body form, the inside will be hollow, which offers multiple possible attachment methods. If I want the mask-head to go on a mannequin or a frame, I tape a crumpled wad of newspaper in the center and stick a length of 1" PVC pipe inside the mask from the top of the head to the "neck", sticking out a few inches. Then I foam it in, making sure the PVC stays at the angle I want. Again, the crumpled newspaper takes up some of the expansion (not as efficiently as a balloon, but I've had no trouble), but I make sure the pipe's cemented in well.
When it's done, the PVC can be trimmed. I slide the pipe that's in the head down over a dowel I've put in the neck of the mannequin or body form, making the mask-heads easily interchangeable. A hole and a cotter pin can keep the filled head from turning if that's an issue, but sometimes, as with the Spider Hill zombie, slight head turning is an advantage.
