Looks good so far, Adam. I would make whatever changes you seek to make and set it up for a test burn before you go much further, though.
If it's like you think it is, a test burn will confirm what you think is so. If not, well, at least you haven't put money into it.
I find myself thinking in the other direction when planning my RMH, such that I think perhaps only the top of the riser will be metal. I don't like the idea of the main piece of my heating system rusting out due to increased oxidisation.
But let us know. I am sure the RMH experimenters will be on to this soon (I have only participated in one build, which, though comprehensive, was a fairly standard batch-box design, and little about it was experimental).
A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects.
-Robert A. Heinlein