Why do you need to fully expose the film? I have some old HP3 that has a very dark fog, but I can get an image from it. Couldn't I develop some before using, bleach it, and presto - the fog would be gone? Wouldn't the rest of the emulsion (ie minus he fog) still work at its "original" (ie pre-treatment) speed? I wouldn't get any better image than before, but I wouldn't have to print through thick fog.
Also, would ferricyanide bleach work for this? Or EDTA bleach?