I think I got it, and I deserve a dope slap for this one. I decided to pull the BB last night, regrease everything, and put it on one last time. If that didn't work, I was going to put on a BB from another bike to see if I could at least eliminate that as a source of noise.
Anyway, in the process of cleaning the receiving threads in frame on the left, I found a tiny shaving of metal. The threads themselves looked OK as did the ones on the BB. I cleaned everything carefully and applied the marine bearing grease more generously than usual and reassembled. I rode reasonably hard in this morning and the noise was absent.
Thanks to everyone for helping me diagnose this. Even though it turned out to be something different, I have plenty of ideas for how I'll track down the next noise that comes along.
On an aside note, has anyone else found the external BB setup to be a bit on the finicky side? 2 of my bikes have external BB's, and it seems like every 6 months or so I'm needing to make a minor adjustment to one or the other to make a noise go away.