Great responses everyone!
A few points:
my thought was you can light the candle and then put it out when you leave. Problem: someone might forget. Possible solution: really short wick that you have to adjust before each use. You could cheat, but you'd have to do something active, passively it will just go out on its own after 5 minutes.
Methane--I'm assuming the quantity that is coming out is not that large--we're not talking about a biogas digester here, just intestinal gas...you know, from all those sunchokes. :) Am I missing something? Willow feeder model? I envisioned this as for the indoor pooper with small bucket you take outside once a week--but still it's a bit of a chill. Coleman lantern seems way overkill, and I wnat to do this without electricity if possible and without possibly-toxic gick.
Toilet seat indoors by the stove--no, the objections raised by the Yukon poster make sense.
Something natural, insulative, easily cleaned even without paint...sound like the birch is the best one so far, but the cleaning part is still slightly dicey.
Squatting--I can squat, but I can't squat over a 5-gallon bucket. I'd at least need some framework around it. Other Americans usually cannot squat. Elderly people would probably not be able to access the contraption.
Great input everyone, keep it coming!
Greg, did you do the test?