Easily modified for more efficiency.

Thermal siphon good ideas ->   http://www.youtube.com/watch?v=5IRLVCJ1olA#t=281

eta: let me know if the hotlink doesn't work correctly... k?

I've thought about this myself. I heard Spirko answering a question about using a rocket mass heater to heat hot water. He warned against putting the coil inside the refractory chamber as it gets upwards of 3000 degrees*. Water hitting that high a temperature would flash into superheated steam. That would result in a catastrophic failure (explosion).
Having been in  Navy, I've had  experience with steam. I wouldn't build anything of the sort unless I was absolutely certain it couldn't generate steam in any way.
I think putting  coil further downstream, after the refractory section, would be safe enough if you didn't run the stove too long.


*  unsure of the exact temperature he quoted, but was high enough to superheat steam.