FAA studied fire hazard of composite skin
Sorry I can't find the reference again - I mentioned it a couple of times in the original battery thread - but the FAA commissioned a study at a university on composite material that iirc was specifically that which was intended for the B787. I have no expertise in the area but glanced through the report and at least did not see anything obviously bogus about the tests or conclusions (that the skin would survive fire well.) Of course, they thought the Li-Ion battery system was perfectly safe, too, so some of the points mentioned up-thread here might have been overlooked.
If anyone is interested in the report and can't find it, ask and I'll dig out a link to it.