UPDATE: After trying many more volume locking apps in the last few days, none of which worked, I finally exhausted all the volume locking apps I could find on the market. So then I took a different tact, and started looking at apps that can create automated profiles. The idea is to forget about locking the volume, and just have the app raise the volume when the phone rings. I tried "Phone Weaver" and "Tasker". Phone Weaver didn't succeed. "Tasker" has a lot more options, and allows you to raise every volume level, when the phone rings. I programmed it to do so, but it didn't work either! Volume stayed the same when the phone rang. I also set it to raise and lower the brightness level to make sure it was actually working when the phone rang. It raised and lowered the brightness of the display when the phone rang, but left the volume intact, even though I had it set to raise every volume going!
It's like my Desire is hell bent on not letting anything mess with its ringer volume! Weird +