Your project sounds great! About 20 years ago I bought the plans for a heating system with the intention of using it for a self-sufficient cabin. Basically, it was a small dedicated outbuilding that was an airtight fire house with a thick sand floor with a grid of copper pipe running through it and circulating via insulated underground plumbing to the house. It used radiant baseboard heat, but you could use a grid cast into a concrete floor for floor heat just as easily. I was going to use a 12 volt
pump to circulate the heated water (actually a glycol mix) because of the ease of generating 12 volts. (Remember, 20 years ago solar was not nearly so popular!) They also had another isolated grid in the firebox to heat your hot water for the home. It was a great looking system and was supposed to run on minimal wood burning as the fire was controlled so well by the airtight box. I believe it was called a Haasa heater or something like that. If you are interested PM me and I will search more for the company name.