The Kang bed is a great idea, it's been around for a few thousand years at least, and been used in Japan and Korea as well.
As for design: do NOT put it the heat chamber above anything other than an insulating base (refractory cement/vermiculite mix or somesuch) because it will leech out alot of the heat and radiate it away from the bed -- namely, the ground, or air if it's the cabin above.
For even more efficient heat transfer, either a set of finned ducts (internally and externally would be ideal) or smaller-diameter tubing embedded in the thermal mass is best (think boiler or radiator design--or, even easier to visualize: I'm floor radiant heating). If the ducts are too large, you lose a great deal of surface area through which the heat can conduct, and out the chimney the heat goes.
That cabin is kinda scary, too: I see so much daylight thru the cracks, air intrusion would render the heating moot.
