It's been a while since I started this thread, so I thought I'd post an update for those of you who gave me advice. It has not turned out well.
As I mentioned the last time, I got the Jon Goodman light seal kit for this Bronica SQ back. I installed the new seals, which was sort of fiddly -- lots of small pieces. I then tested the back again -- the film still showed light leaks, same if not worse.
Jon was helpful guiding me through the process, and when I wrote to him that the repair wasn't successful, he generously offered to look it over. He not only looked it over, he actually replaced some seals that he thought weren't quite right. When he sent it back, I found the seals were actually now in better shape than the other Bronica back I have, which doesn't leak at all.
Yesterday, however, I shot a roll of film using the back, and still it leaks. It's true it only does this if exposed to bright sunlight for a while. The parts of the test roll that I shot in shady streets showed no leaks at all. I wrote to Jon again, and he's just stumped.
So I guess I'll have to live with it. I'm going to try putting a women's elastic headband around the join where the insert fits into the holder, hoping that might do the trick. Thanks again to all of you who offered suggestions.