This is interesting, kind of a variation of what I raised in post #18, except I contemplated developing the unexposed,fogged film first. But doesn't developer just act as an amplifier? Perhaps Fanshaw is suggesting something like just bleaching the unexposed, undeveloped film to treat the miniscule amount of latent silver, unamplified by development?
Would the permanganate have been used with sulfuric acid or sodium bisulfate, as used some times with reversal bleaches?
And what about using a rehalogenating bleach instead?
I may try this with my old FP3.