First of all, I own 2 XPan II. Second, I have also cropped Leica M9 image down to XPan dimension, and yes under some good conditions, it can work. Of course my requirement is to be able to print at least 20" wide so if your requirement is web-pics or smaller prints, then there is no problem.
A P30 digital medium format back crops much nicer, just saying 🙂
In any case, XPan is not something that you shoot "once in a while." Sure it's fun once in a while, but to grok its point of view, you must stick with it, until you start seeing the framelines floating in front of you 🙂
As for modding 6x9/6x7 cameras, there are people on EBay who sell 35mm sprocket kits for medium format cameras. Probably the best way to get started. LOMO's Sprocket is ridiculously cheap, but only if you like plastic camera imaging.