I was talking with a friend of mine yesterday about rocket mass heaters, and showed him some links and videos. And he immediately said 'Why don't they cast that bottom bit out of CO2 sand?' I explained that various attempts have been made to cast the core out of various things, mostly without long-term success, but that I'd never heard of CO2 sand.
Apparently it's used in making molds for casting engine parts and can take some very high temperatures. It's also being used as a building material in disaster relief areas.
I've collected a few videos and links and just dumped them here for brainstorming purposes.
I think the answer is in the forth video after the casting the chap pokes near the pattern and it falls apart. In the second video it is "hard as brick" but they are using epoxy to stick the sand because hard things tend to break easily as they are not ductile. The ductility here comes from the epoxy which I would think would combust in the rocket mass heater. If you could replace that with some else like a e.g high temperature silicone? maybe it would work.