Howdy Will, welcome to permies!
I am no expert, just learning myself. But I will take a shot at an answer based on what I have read here at permies.
It seems to me that a firepit and a RMH are two different things. The rockets need to be enclosed and built to certain sizes to reach high temps and burn efficiently. They then are used to heat the benches. I am not sure how an open campfire would fit into this? What kind of an idea do you have?
Also be careful with hot water systems, they tend to flash and explode!
Have you seen the new DVD's that permies is selling? Lots of info about RMH's there.