Lube those hinges!
I found the problem I think, and thought I'd post it, in case others encounter trunk lid problems. I could see no signs of damage to the hinges (car has had minor rear end damage), so I watched closely as each hinge opened & closed. The left side top-most hinge pin had over 1mm axial play in it, where the right side had no visible play. 1mm = 3mm or more at the front edge of the lid. There were signs of rust indicating poor lubrication. Not sure what I can do about this, short of a new hinge, or possibly pressing in a new pin.
So... lesson learned: Lube those hinges! There appears to be significant force on the upper-most pins just as the lid is raised, and the front end of the lid wants to dip down at this point. If the pins wear, the lid will dip down and the corners will contact the edge of the rear window. I can't see any way to adjust this out once the pin is worn. For now, I've put a rubber bumper on the lid edge to prevent contact with the glass.