I did use some metal in my RMH, but it could be avoided easily. I have a masonry and cob bell, which could be done in all cob (but with less safety and code compliance).
Rocket mass heater with 8" J-tube and bell
My steel feed insert could be replaced with a firebrick feed easier and cheaper... I wanted to try it for the p-channel supply and air preheating; it works very well, but is not essential to operation.
An access panel to inspect the riser is a really good thing, but could be done much more simply than mine if you are not concerned with "living room" esthetics.