Tonight I took the bike out for a 15 mile round trip in the freezing cold pouring rain (which I wasn't expecting but that's another story).
Lights and engine ran normally. No electrical trouble* of the kind I was having. Assume I solved it?
*I did have one issue: intermittently, pulling in the clutch is triggering the high beam and the high beam indicator. But I don't think that's related to my electrical problem. If it continues I have a couple of ideas: either the bar mitt is slightly deforming the angle of the clutch lever, or when I gently laid the bike down on its side unexpectedly last month (nothing interesting to tell, I got unbalanced while working) I bent something in the lever housing