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How about not modifying the gearing of the back, and instead of cutting a centered hole in the darkslide, how about just cutting the darkslide in half? Does it go in from either direction? If so, then just swap it out to the other side after exposing the first half of the film. People do this (longitudinally) to do 4x10 images with an 8x10 camera. E.g. a half-darkslide splitting the film into 2 4x10 exposures.
Just a thought. Not sure if it's feasible or not but it's easier to try than modifying gearing.
Another option (albeit way more work) would be run the film through the camera twice. the 2nd time around, advance another half-frame beyond the start point, so the new images are offset by the 28mm or whatever the width of the image is. Requries rewinding the 220 film though = darkroom or changing tent. Much more hassle but doable without modifying the camera (some trial and error required likely).
Just some ideas...