Good idea as usual, Dale!
For char, a retort might be a major hassle. A friend of mine is using what he calls a ring of fire to make char, though he isn't using stumps. A metal ring (his is five foot diameter and a couple feet high.) build a fire in it, and feed till you have a big pile of embers. This can be put out with a remarkably small amount of water -- he is using a backpack sprayer to do that. With a stump I'd do a small fire on top of the stump, make sure the ring is taller than the stump(s). Keep building that fire outward till the ring is full of fire and then till the top of the stump has dropped some. If it turns out you haven't burned the stump all the way through, use what's left for hugel base any way. It will partially rot, and the rest will become biochar over a very long period. That's the plan for the movable stumps on my place, anyway.
Somebody cue up Johnny Cash's "Ring of Fire." Don't forget the marshmallows and hot dogs!
Gani
Intermountain (Cascades and Coast range) oak savannah, 550 - 600 ft elevation. USDA zone 7a. Arid summers, soggy winters