I think it was a few years ago I went up to The Fort at Grand Portage for the festival activities, and that is where I noticed cooking the fish on a slab of rock.
I also noticed the boiling water as you mentioned but they didnt use a bag they had a bowl made of birch bark.
If a birch bark bowl was filled with water/soup and suspended over the hot coals, Id think it would catch on fire. Guess it depends on how close it is.
They had alot of games they played there also. The returning champion of several years was back to defend his title on how far he could throw a heavy rock. He won again.
They had relay races, of which part of the race involved a paddle, and a shot of brandy and balancing. Well you get the idea. ;) Fun times at the Grand Portage!