At what point, if any, does fire safety trump permaculture?
In my opinion, point zero.
But, I don't see a huge conflict of interest here. 100ft of space isn't all that hard to have while still practicing PC. In my defensible space are water collection tanks, a vegetable garden, flower beds, a tiny pond, a couple shrubs, bushes and fruit trees (not within "spreading distance"), storage sheds and the not yet operational chicken run. None of these raised the insurance inspectors hackles. Still, where I live if a "real" fire comes through there isn't going to be much anyone can do about it, defensible space or not
I sorta think that a well managed food forest is basically what you would get after a fire came through anyway. Good soil, healthy spacing and an effective succession system. Isn't that why the fire danger is so high and the forest health so low? No fire for 100 years? Just from looking at the difference between a well-developed food forest and the average (and also well-developed) tinderbox forest here in California keeps me working towards one of my own.