With the jack in hand, I idly poked a coaxial plug into it and realized that the amount the plug stuck out was just about exactly equal to the thickness of the black plastic cap on its tip. Some rummaging turned up one of the six plugs with a missing tip, at which point both the problem and its solution were obvious.
A bit of tedious work with a tiny screwdriver and a needle convinced the socket to disgorge the plastic ring from its bowels …
Now, I suppose I could have figured this out without taking the case apart, but actually fixing the problem would still require surgery, soooo there’s no wasted effort. That’s my story and I’m sticking with it.
If you think you could extract that ring from the outside, there’s a joke about that.
I put the case back together with a few dabs of silicone snot adhesive (despite what I know about letting acetic acid loose near electronics) to anchor the circuit board, applied a belly band of tastefully color-coordinated (i.e. silver) duct tape, and it’s all good.
Actually, the pack was stone cold dead until I plugged it into the charger to reset its battery protection circuitry. Evidently, disconnecting and reconnecting the battery tripped the protection logic. I’ve seen that in other Li-Ion packs, so it wasn’t quite so scary as it was the first time around.
As for the coaxial power tip: a dab of solvent glue, an overnight clamping session, and I think it’ll work fine forever more.
I should machine up some stabilizing collars around the sockets to match that obvious shoulder on the plug, shouldn’t I?