The wool insulation people (from my earlier post) responded to my email very quickly, and I also inquired to Stefan about its suitability in an X-house. He replied that it was very suitable, and that there was 300mm space available in the floors and ceiling for insulation.
We've been investigating other eco-friendly insulations as well, and there is a diverse marketplace out there, to be sure. Be prepared for more on this subject! Who knows, maybe there'll be as many insulation posts as there have been on toilets.
I plan to talk to the people in Wales soon about details, and I'll report back.