The hydronic systems could be really dangerous, but there are expansion tanks, temperature and pressure security valves, etc. The problem with the rockets is that there is no shutter to automatically stop the burning process before the boiling temperature is reached. What came to my mind is something like a well insulated "aprovecho institutional stove" http://www.aprovecho.org/lab/institutional-stove-project/institutional-project with a copper coil inside the "boiling" barrel to transfer the heat to the hydronic system and a flue dumper to stop the air circulation after the burning has stopped.
An open non-pressurized system which includes a blow out pipe for steam would be quite safe but would need more management since you would be sending hot water down the drain any time the boiling point is reached. The water level would need to be topped up occasionally and an open system would allow oxygen to enter the system which could cause a greater rate of corrosion of metal parts.
Since you can draw water from your open system the same tank could be used to store domestic hot water and it could be topped up with a solar batch heater. More length of in floor pipe would be required because you'll usually be operating at less than 200°F. An open system may be impractical for a two-story building.
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