Hmm. Tinfoil might be the better option for you then, since you could bake the sculpey right on the tinfoil.
OH! OR you could cover his head in the plastic wrap, spray a little Pam on the plastic, so the sculpey won't stick when you remove it? I'm not sure what the Pam would do to the inside of the mask, but I would assume it would just bake off? It might be worth a shot doing it with a smaller piece to experiment and see...