Have you thought about thatch roofing? The phragmites used in the UK for thatching is now spreading like a pest here in the US, choking out / hybridizing with the native sub-species of "water reed". It takes a lot of labor - there is a book (scanned) on a UK historical preservation website which tells/shows how to properly thatch a roof - starting with framing scantlings. A good "water reed" thatch job can reputedly last 30-50 years before major work need be done - some roof have been found with reeds more than 400 years old against the roof framing! Small grain straw was also used historically in many districts for thatching, but doesn't last as long as phragmites.
Yes, fire danger is an issue. It can (apparently, again no personal experience) be somewhat mitigated by plastering/sheetrocking the inside of the roof space and keeping combustion processes out of the attic - even being careful with electric lights. A good chimney spark catcher and lightning conductors would also be beneficial, I suppose. The thatching guide I mentioned can be found here:
There is some mention of good fire control practice in chapter 7 (download "Part 10", start at page 9). Inexplicably, framing detail are in Chapter 8, which comes after Chapter 9 in "Part 10"? I recall having seen mention of fireproofing solutions to be applied to thatch, also, but anything other than Borax would seem to be "iffy" to me from an environmental standpoint, and I don't know how effective Borax really is as proof against fire.