I don't think FLW used
rocket mass heaters. I believe that he did utilize
Masonary Heaters. i remember hearing that he used under-floor heating too. a quick internet search turned up this wikipedia page:
http://en.wikipedia.org/wiki/Underfloor_heating which contains the following account of the origins of Wright's underfloor system (also a more thorough definition of ondol heating which evolved into modern radiant floor heating):
In the early 1900s, when the American architect Frank Lloyd Wright was building the Imperial Hotel in Japan, he was invited to the home of a Japanese nobleman. There Wright found a room that was different from typical Japanese rooms, with a warm floor covered with yellow paper—a Korean ondol room. The Japanese gentleman had experienced ondol in Korea and, once back in Japan, had an ondol room built in his house. "The indescribable comfort of being warmed from below" impressed Wright.
Wright decided then and there that ondol was the ideal heating system and began incorporating it in his buildings. Wright invented radiant floor heating, using
hot water running through pipes instead of hot air through flues.