We did install a couple of Cozy Heat in a two of the superinsulated (R-50 wall) homes I built in the 1980's but the owners did not rely on them entirely for heat, although they put out a considerable amount. My good friend Tim, southwest of Denver (Elv. 8,000) installed a Temp Cast in his new log home but this is recently so I can't vouch for the effectivness of it in this context as I have not been back to see it operate.
The "Masonry Heater Association" is worth a look, I would not discount this system in Maine in a Passive House as you will need some auxiliary heat in the winter months. The masonry heater can be sized with the appropriate thermal mass to store the right amount of heat for a given heating load, as the weather warms, you build a smaller fire. I think it is critical going forward to utilize the local (renewable) heating fuel, wood, in the best possible manner, this may be the only heating fuel available in the future. As I understand it, a masonry heater can be set up with a coil to provide domestic hot water as well. A Passive House uses roughly .6 Btu/sf/hdd, same as 15 kwh/M2 annually. Don't know the design temperature where you are but where I live, Minneapolis it is - 16F, so on the coldest days of the year a 3,000 sf passive house is going to need about 150,000 Btu's per day to maintain indoor comfort. This a therm and a half or 25 lbs. of hardwood at 75% efficiency, not trivial.
Mark, the pellet stoves do look interesting, I have considered pelletizing and burning native (prairie) grass in the future as I have this resource on my farm, has about the same energy per ton as quality firewood.