Anonsi, I'm convinced about "hemi". A mere prefix is not acceptable, but the use of "hemi" for a type of engine qualifies it.
"Demi", apart from being a prefix, seems to be an alternative ("now rare or obs." according to the Shorter Oxford) for "demy", meaning a size of paper and various other things. "Demi" is also the plural of "demos", meaning a district in ancient Greece or a personification of the populace. ("Demos" is not currently in our word list, but only because it could be the plural of "demo".) So, all things considered, "demi" should be allowed too.