Yep fire brick is good.
I was thinking more of this being a cook stove where you would want the mass heated fast in a limited time, 30-60 minutes of cooking, then get that extra heat back out before the next meal is cooked for another 60-90 minutes or whatever. More of a fast heat, fast cool setup to even the temps in the room between meals. Also I figured the cast iron stove being hot to begin with will only cool at a set rate, enternal mass like copper should not cool faster than the surrounding cast iron. Well, provided it does not have a draft of cold air flowing over it
I am thinking of heating something in a wood burner I mentioned earlier I think, that I can carry upstairs in a rock house to hold/move heat. Not sure what I could use for that. Thinking again on the aluminum scrap, heat it fast and hot at the fire then sit it in a tank of water upstairs to heat the water which will slowly release heat over night. Maybe about a 20lb hot block would heat the water pretty well in 2 trips?
I'll use a heat exchanger and water tanks downstairs probably, but can't upstairs since I won't be able to run the pipes
Something that gets real hot, is light to carry, won't spill, and will heat the water fast that I store upstairs is what I'll need for this. Water will be solar heated durring the day, so I mostly need to top it off at night before bed with extra heat to last till 6am.