I have heard of Modern day Locals in Idaho, covering the animal in a clay based mud. The skin is left on and inerds left intact. Set in the embers and then covered with more coals, cooked for hours? ( Don't know cause I never did it or saw it done ) But it was told to me that the clay is hard and broken off. The skin stays attached to the clay. The insides are not eaten only the meat. This sounds plausible but quite primitive. Steam or moisture is supposed to be trapped somewhat, and assist in the cooking process.
I can think of easier ways to cook, and I like seasoning and spices.