Great thinking on your feet! I've also seen a rocket stove water heater that uses a tank from a water heater. A copper pipe comes out of the bottom of the water heater, goes to the chimney of the rocket stove and tightly coils around that upwards. Then back into the top of the water heater. Then when the strove is lit, the hot exhaust heats the coils, the water is pulled upwards and heated. it was pretty ingenious. It's on youtube. I'd link it, but I'm working the ER tonight and video sites are blocked...