The underexpose + black backing trick should work. It's going to require lots of experimentation though. Thinking about it, why not make 3 or 4 exposures on one plate by removing the darkslide partway, exposing, moving it again, exposing again, etc. That'd get you a series of steps of exposure, each one stop more than the previous. (You'll save plates too).


As to reversal, I've used dichromate & ferricyanide bleaches. The dichromate is nasty, though once its in solution its much easier to work with as there is no dust issue. The ferricyanide bleach tends to soften the emulsion much more.

You could also just contact print the plates.