Actually the new technique we tried was to put a rock on the fire grate, place the brownies in a pan on the rock, then cover all that with the big cook pot.  The theory was the inside of the pot would get hot from the rising heat and the rock would protect the brownies from the direct heat of the fire.

It did not work.  The pot never got very hot so we ended cooking the brownies for 3 hours.  To make matters worse, we over cooked them (they probably were done after ONLY 2.5 hours) but the result was brownie jerky.

I like your idea.  I guess it is basically the same principle, using the water to protect the brownies from direct heat.