We do something like that with a crock pot, and a dutch oven, all the time. I realize that the crock pot required electricity, but I mention it here 'cuz it works so well. With a smaller round crock pot, a Wallyworld grease pot works perfectly, in the larger oval ones small loaf pans (get the non-stick surface ones for ease of cleanup). Rather than a Mason jar ring (which I didn't think of), we used three large (3/4" or so" stainless steel nuts (as in nuts and bolts). Arrainge them properly, and sit the pot/pan on them. Real easy is Jiffy biscuit/cornbread mix, but you can use your own mix as well. You will have to experiment with cooking time, and may be surprised, as we were, to discover that the biscuits will actually brown on top. We even stack the loaf pans, using a couple of pieces of coat hanger wire to place one pan above the other. We have even made two pans of different stuff at the same time. More experimentation requred for times/temps in the the dutch oven. My wife makes killer biscuits that way, in a round bread pan within the oven. Works like a champ...

ps: we do NOT usually cover the pot/pan within the crock/DO, but I guess that could depend on what you are baking...