dude... you can right-mouse click in the windows environment, and if you load the MS (and, perhaps, the other guys drivers/mouse combos), you can right mouse click in both OS9/X.
I have a Belkin USB KVM switch and use two PCs, one Cube, and One TiPB. No problems what-so-ever. I use a PC keyboard, but have plugged in the Mac USB keyboard and it works fine -- except you don't get the "windows" keys.
One challenge that exists with my scenario is that Mac OSX locks up my KVM from time to time. All four machines are booted and running just fine. I can switch between the windows and the OS9 machine all day long with no problems. And I can switch between the OSX box with no problems most of the time. Sometimes after switching away from the OSX box (the Cube), the KVM fails to recognize the mouse/keyboard on the other machines.
Damn. I've talked to Belkin at length on this and they haven't admitted it is a problem, but do suggest that there is a firmware release for the KVM coming. Not here yet...
:->