Well this went an entirely different direction than I had expected.
I replaced the fuel pressure regulator and all of the lean issues seem to have resolved. I've put about 2 hours on the truck since then, with absolutely no indication of a problem.
Two different Ford mechanics told me that the pressure regulator had tested OKand in good-natured fashion laughed at me when I told them that I was going to change it. "That's a waste of money. The regulator tested out fine". (3 fuel pumps and 3 filters later, I replaced the $30 part.....)
Apparently, they're not just pass/fail as long trim now varies from -3 to +3, where it was locked in at +25 before.