At one point along the way, the Control Panel reported the ABP temperature as 1024 °C, which seemed excessive. A bit of poking around revealed this situation on the ABP connector:
The connector just barely clears the top of the X axis homing switch board and the loose wires tended to rub on the top of the cable connector. I’d been meaning to fix that for a while, but now I had a real reason.
A bit of soldering and some self-vulcanizing tape later:
Also: notice the discoloration on the connector shell surrounding the Black wire? That’s the contact leading back to the MOSFET from the platform heater: a single pin carrying far more than its rated current. The shell around the contact on the Red wire (which carries the same current) isn’t discolored, which suggests the Black connector is a bit loose / poorly crimped / whatever. It looked OK to me, so I left it alone.
While I had the cable on the bench, I added a set of those right-angle pins to eliminate the risk of loose wire ends getting into the wrong places.