This proves to help things a lot. I managed to blow through the major functional issues today, and CommYou now appears to be entirely workable in IE6. There are some cosmetic blemishes (for instance, the loading screen looks like crap, and there's a weird border around the menus that shouldn't be there), but I'm not going to worry about those right now. In cases where the cosmetics got really problematic, I've generally hacked around them. For instance, the helpful "here's what to type here" OverText for the text-entry boxes was so badly misplaced as to be less than useless; those are simply disabled in IE6.
I might yet relent and pretty it up sometime later, if I decide the market warrants it. But for now, taking a moderate approach to IE6 seems to be best for my blood pressure -- I'm not losing a week on pixel-twiddling, but I've got it to the point where I can say, "Yes, it's still usable even if you can't upgrade" with a straight face. I am *very* happy that I got there in one day -- I was expecting much worse, but many of the IE6 problems are mainly about getting the look-and-feel just right.
(Of course, I don't yet know how it works in IE7. But that's a project for another day...)