I can't give you an accurate answer from experience as I only have 120 backs for my Koni. Thinking through it though, the only risk should be that the frame spacing would be a little wider due to the 120 film and backing paper rolling up thicker than 220 film. You might loose some of the 10th frame.
I don't think that you would run into any focus issues because of the way the pressure plate works in the back. The pressure plate retracts from the film gate when you wind, and then presses down, holding the film against the film gate for the next frame. There is a bit of spring tension holding the pressure plate against the film gate so it should hold it snug. It should still pull away enough when winding so as to not cause problems.
Obviously, you'd have to wind past frames 11-20 at the end of the roll. I suppose that there could be some issues with lining up the first frame as well, but I would think that it would just start a little later on the actual film, which again would contribute to loss of some of the 10th frame.
Again, I have no direct experience with the 220 backs so I may have missed something. If you do try it, post back what your experience was.
