Hmm...this may be a dumb idea, but my understanding is that the deep litter method helps with heat because the bacteria and whatnot that's eating the organic matter in the compost give off heat as a byproduct. Sooo if you were able to start some compost in a heated location, let it get going really well, then take it to the coop and bury it in your existing "deep litter," mayyyyybe the compost germs would keep doing their thing, colonize the deep litter, and provide some warmth?
You'd have to decide if the experiment is worth having stinky compost in a tub somewhere though. And this is just me brainstorming...NO idea if it would work, especially in lows like you experience. You may be better off just adding more insulation to your coop or something.