Okay, so new development, as all my creak reduction efforts have ultimately been unsuccessful. At this point I would take it to the dealership, but I'm getting married in exactly a week and don't have time to mess around with the dealership until after the wedding and honeymoon are over.
As I've mentioned, this sound appears only when it's hot out. I had previously realized that I could make the sound go away by tugging the headliner down from the leading edge by the windshield, the passenger grab handle, etc. In other words, by relieving the pressure of the headliner on the roof. I haven't been able to rig anything to keep the pressure relieved, however.
Anyway, it's hot today and the thing was creaking away, so I pulled down the weatherstripping and tried to get a hand in there where the sound is coming from. I also peeked up in the headliner, and noticed a black gooey substance in the limited area I could see, and I think I've realized what the sound is and why it only happens when it's hot.
I believe when the weather gets hot enough, the adhesive attached to parts of the headliner semi-melts and gets gooey (kind of like tar). I further believe that the sound I'm hearing is actually the contour in the headliner couple of inches in front of the grab handle sticking and unsticking to the gooey headliner adhesive as the car twists, goes over bumps, etc. When the weather cools down, the adhesive re-solidifies and presto, no more noise.
Anyway, I didn't have much more time for troubleshooting, as I should have been at work this whole time, but I'm going to redouble my efforts once I get out of here for the day.
So no solution yet, but I think I'm getting close, and I think that if I were to take off the A- and B-pillar trim, I could fix this relatively easily.