Passive Heating and Cooling

Here's an interesting design which demonstrates some key principles of passive solar heating an cooling. It mentions the slate floor which acts as a thermal mass (slowing heating and cooling over the course of the day and night, respectively.) One of the ideas we've been tossing around is using the moss wall as our solar mass, as it's density may be appropriate. (We may want to consider adding a thin stone tile backing to it...)

I've also been reading the Humanure book online, and found the temperature that compost can reach- up to 70 degrees Celsius. This is pretty exciting, as we were also discussing an idea for a water heating system which would run a coiled pipe through the compost containment unit, and then through the central wall of the house. This would heat the house, as well as provide hot water. Any thoughts on how feasible this might be?