OK, you learn something new every day! We tried Modtron's procedure, and it works like a charm! Reversing a channel "on the fly" is not something normally done in aircraft, so the reversing function is what people would normally use. Of course, you can't use it while flying.

We used Mix1 and Mix2 in our 9C, and turned trim on and link on on mix1, the same on mix2. Both were set up with -100% for both ends of travel, and we put them on the same switch with "down" as the activating direction. When we flipped the switch, the channel's travel was reversed.