Yeah I'd be shocked if he wasn't used inn Django Unchained. QT seems to like meshing old score pieces together in his films rather than hiring a composer. It is an interesting way of putting together a soundtrack. Although I would love it if just once he did work with a specific composer. It would be pretty amazing I think.
He did, sort of, in
Kill Bill, vol. 1. "Battle Without Honor Or Humanity" is an original score, which the artist (whose name escapes me) offered to write for Quentin--for free. Quentin's interest was piqued at the idea, and he agreed.
When QT heard it, he loved it--and put it in the film.
On Ennio...I recall he was going to do the score for
Inglourious, but for whatever reason, couldn't. So Quentin improvised, using some of his previous works.