Mucking around with the connector and wires a bit while moving the throttle by hand eventually produces a seemingly running engine again, and we manage to then drive back to Westford MA without further incident.
Problem seems pretty clearly local to the throttle position sensor or it's connector, so instead of going to a shop and dropping significant $$$ I called up Auto Zone and asked if they could supply me with a replacement sensor for my xB; they had one in stock and I borrowed L's xB and picked it up. Duralast part number TPS4117. I was wearing zero pieces of flair but they sold it to me anyway.
Anyhow, it's in a hard to get at location without removing other shit, so I had to take out the air intake stuff in order to get at the connector enough to disconnect it and remove the sensor: but I dike out the air intake and air filter holding enclosures until I can have good access to everything, whereupon I find this:
Well, golly, that yellow wire looks to be in *great* shape! (Not). Ok, I probably don't really need the new sensor, I actually need a soldering iron and some heat-shrink tubing.
Cut wire at broken location, stripped, soldered, heat-shrunk, etc, and reassembled everything; everything proceeded to work just fine and no more engine codes. Drove it back to Auto Zone and returned the unused sensor. Yay!
I wonder what various shops in the area would have told me, and what they would have charged for the service. I wonder if *any* of them would have simply told me about the broken wire and just fixed *that*.