Thanks for the input Tom but like I said in the original post, what I'm looking for is simply confirmation one way or the other from someone who's done the mod. If I wanted to just buy another holder I would (and I still may) but this thread is about the particular mod I've brought up. (I opted for the Horseman because the Graflex holders are inferior with established film flatness issues. The Horseman is a superior back that I would like to use if the modification indeed solves the issue at hand.) You're absolutely right that the film plane is what matters so assuming that the film plane in the Horseman matches that of the Graflex, then the mod should work and that's what I'm asking about.
So back on topic. If anyone has done this mod please chime in. Thanks!