Okay, I got some 20A fuses and did some testing.
1. The fuse didn't blow on the parking lights, headlight, or engine start. I rolled the handlebars all over the goddam place, bike stayed running. (I didnt get on and ride since I didn't have my helmet.) So if it's a lighting fault, it's an intermittent one.
2. I removed the headlight and poked around in the nacelle with a strong flashlight. It's a rats nest, but nothing appeared obviously exposed. I've already worked back there (a year ago when I fixed my "bike shipped with gerbil nibbles in the rectifier circuit" issue) and had previously done some light rerouting and rebundling.
3. I removed the two tank bolts (neck and underseat) and looked carefully at the routing of wiring under the tank and especially under the neck flanges. I found a couple of dicey stretches I wrapped with electrical tape, but nothing that was obviously the source of a fault. Under the front right tank flange there was a plastic tube containing a bundle of wires that was pressed against a bolt head, so I rerouted that a bit and reinforced it with tape. Similarly on one of the wires on the right rear of the tank. (The throttle cable housing in that area is definitely rubbing, and I reinforced that too, but it's not the cause of my fault.)
4. I removed the battery cover and checked carefully above and behind the battery, focusing on the positive terminal. I removed and discarded some obsolete accessory wiring in that area that was in the way. I taped up an exposed area (about 1mm) at the very end of one of the wires leading to the positive battery terminal, just in case it was shorting on something.
5. I looked at all the wiring from the underside with a strong flashlight. There was no evident issue (aside from filth) with the wiring under the rear fender leading to the taillight.
6. I put everything back together and parked the bike. Later I'll take it on a test ride (a couple miles in loops close to home, so I can push it to the garage) and hope for the best.
Oh, and I'm carrying 8 spare fuses on the bike...