In reply to murray:
Cairngorm Mountain tried this for a while in the late 2000s - the Cairngorm Poo Project, I think they called it. At one point, you could collect poo bags from the ski centre, and then later on they came with a reusable plastic pot, presumably to stop the bag from exploding in your rucksack. There was a chute to deposit used bags into, also at the ski centre.
I don't know when this project stopped running, but I can't find any references to it after around 2009.