It seems that it was a good idea to do Cornelisz's head and armature first; he's got a beard, and a great deal of his head is covered by a hat. So it's comparatively easy to cover up suckage.
I've also solved another problem; up to now, i've been endeavoring to cast the heads in two separate halves - that means that i'll end up with a prominent line where the two halves meet on the finished thing. Not a problem for this head, as the beard covers it. But none of the other characters have beards.
I've found that I can cover the entire head with liquid latex (thereby taking the impression with one piece only), and then peel the whole thing off inside out.