I did a simple modification to a spare dark slide for my RB67, cutting about half the material lengthwise. (there was a url link here which no longer exists) Some other contributors to that thread made some useful suggestions regarding actually converting a back. Anyway, the dark slide trick gives me two individual ~26x70mm frames per standard 6x7 frame, and I am free to shoot a whole 6x7cm frame at any time. When composing, you just have to remember that the top of the viewfinder image is the bottom of the film holder, and vice versa.
Also, I want to convert a 220 back for my RB67 into a 35mm pano back, but this is still one of many planned projects sitting in a box full of spare parts.
Some have said that they don't see the reason to do any of this, but I learned to shoot on slide film, and was taught to crop in-camera. As a result I like to compose for the final print, with minimal cropping. The pano dark slide gives me that option without carrying (not to mention BUYING) another camera.