Since I could not reproduce the symptoms of a sticking solenoid on the bench, I decided to look at the wiring again. I left the battery cables detached, and reasoned that if the ignition switch were hanging, or otherwise keeping the blue/yellow wire to terminal 50 energized, then I would find continuity between that wire and the positive battery cable: I did.

Aha! It's the ignition switch! So I pulled the connector and checked again, and sure enough - no, wait . . . I still had continuity. Even with the ignition switch disconnected, I had a reading of something like 32 ohms between the blue/yellow wire where it connects to the starter solenoid, and the positive battery cable. Something seemed wrong, so on a whim, I checked the continuity of the blue/yellow wire to ground (starter off the car, blue wire hanging free): almost no resistance to ground, like .2 ohms.

I lost daylight while I was puzzling this, but I did decide to move the gear shifter back and forth, and check again: next reading found something like 1kohm between the blue/yellow wire and ground, and about the same resistance to the positive battery cable.

So for the moment, I am operating on the assumption that the neutral safety switch ("start inhibitor") is the culprit. Never heard of a failure like this before, but I will surely let the board know what I find. I am a little worried about what might happen if the blue/yellow wire were to ground - from the looks of the wiring diagram, I fear that might mean the starter motor would run backwards . . . not a pleasant thought.