Damn. Looks like I'll be seeking out some 220 film. I'm willing to try 120 because there were some firm believers that this would work fine, including my KEH sales rep. I'll have to learn for myself as nobody seems to have concrete examples of this in practice. Thanks for weighing in!
Oh, I've tried it as an experiment. And it worked fine. It is just that there are concerns about:
1) being misled by the counter - I rely on the clear indication that the 120 back gives you when you finish the roll; and
2) that heavy use may cause accelerated wear on the back.
In addition, the spacing is a little bit different. If you use 120 in the 220 back, you may want to experiment a bit with the "Start" marker on the film in order to achieve the right amount of unused film before and after the shots themselves.