I've seen a few different RMHs used to heat water (for a
shower system) and the key to me would be to have an open system, not pressurized. A pressurized system is just one failed pressure valve away from BOOM-SQUISH (death due to explosive force and/or steam scalding). Then run your water lines through that heated water, keeping the water in the lines totally separate from the water you heat directly (sort of like a double-boiler?). But if the water-flow in the pipe stops while you still have an active fire going, then that section of pipe in the tank will be heated up quite a bit, and I'm unsure how you manage that heat buildup. That seems like another single point of failure, if that pump fails. I expect a person would try to automate the
wood feed or at least use a batch box, and that automated or unattended wood burn could lead to the water being overheated.
Wheaton Labs has a water heater where the J tube heat riser sits under a water tank and heats that up, and then a water line runs out to showers. It has a thermometer and those who fire it know how much wood to feed it to hit a safe temp. I would be incredibly cautious about any direct water heating system, making sure the water never gets close to boiling temperatures. The imaginary lawyer in me also says "none of this is intended as professional advice or recommendations, any risk you take with such a system is yours alone and could result in injury or death".