Having come late to this thread, a few comments on the recent posts I've managed to read:
"that MR4 should be made as safe a is humanly possible for todays standards"
Well, it'll never fly then. That'll be as safe as we can make it! The best you can hope for is as safe as is reasonably practicable. That puts it into a judgement call of what you can reasonably do, and indeed what is reasonable.
"OBIGGS is a one shot system if it fails to put the fire out in the duration of the nitrogen charge....,"
Err... not quite. It Generates the Inert Gas over a period and once the required concentration is reached it should prevent a fire from starting. If the volume, e.g. bomb bay is so leaky, which it sounds like it is, that you can't build up the N2 content, then you'll never prevent a fire, which would be agood reason for not trying to use it in that environment. There are however several other systems relying on other extinguishants which could be used in that sort of environment and could be activated very (very) quickly by flame detectors or not quite so quickly by firewires.
However although I have to agree that fuel shouldn't be leaking in the first place, it should be a risk that is foreseen and measures put in place to deal with it....if these are reasonably practible.