I think i've solved the problem; what i've been looking for is a flexible material that I can use to take an impression of the clay head; flexible so that I can remove it even with the undercuts (if I cast it in plaster, it would get stuck). I just used the same liquid latex.
The stuff has different working properties than I thought; I applied very thick poured-on coats, and dried them partially with a hair dryer. Then I put more on. That is to say, I was able to build up a very thick layer in one evening. When I was making the rubber mask for my previous failed attempt, I was putting on very thin coats and letting it dry for half a day. You can really go hard with the stuff, and it will still cure & dry evenly.
So now i've got a complete rubber mould of the head. I'm going to take a test cast of the face area with alginate to make sure the details transferred correctly. Then i'm going to make the rubber mask, by pouring latex inside the other one, swishing it around, and draining out the excess.
Boy, i'm getting pretty sick of this crap. I'd like to begin making a film already.