If you want torn up nasty burnt flesh, do a latex build up over the endoskull.
If you want Ahnold with his flesh half ripped off, exposing the endoskull... You'd have to make a copy of the skull to make a core, sculpt on top of that with clay, mold it, run silicone, paint, and transfer that to the finished endoskull... And I am way over simplifying that process.. it's definitely not as easy as just wrapping it in some magic flesh material though sadly!
I recently watched a small featurette on the BBC about an artist who makes prosthesis. She has a fine eye for detail and colouration. Take a look at her realistic skin setup. http://www.bbc.co.uk/news/magazine-30551860 starts at about 2:58 but it is well worth a full viewing.