While doing some additional research, I found this company. Looks like an interesting product. Not sure how it would work with a gasification stove as you cannot steal too much heat from the stove and it still operate efficiently. I am wondering if this could be inserted into the exhaust path and capture less heat but ensure the stover operates at optimum efficiency.
Therma-coil wood stove domestic hot water heat transfer coil
Traditional thermal transfer coils for stoves run on the outside and capture radiant heat, which is far less efficient, but does not pull heat from the combustion chamber. The enticing idea I see with this product is the ability customize how much transfer area you have inside the stove. With a super hot combustion chamber, you can expose just a small amount of tubing to the water and transfer a lot of heat to the water system. Though I do worry about the safety aspect of pressure building. I suspect I will have multiple blow off valves in the system for pressure, so it should be fine though I guess.
I like the idea that the water would heat faster as well. A direct fire heat source vs radiant coil has to mean faster heating time for the loop system.