It's probably bad form to reply to my own thread, but for anyone searching for this, here's an update:
Bike is a Motobecane Le Champion with a titanium frame from
BD.
I took it to my mechanic's place and we removed the BB. The threads look to have been greased with a miniscule amount of Mao-era surplus army grease when the bike was assembled, so we greased them with a generous amount of Ti-Prep and reassembled everything. It was not easy to remove the aluminum bearing cups after only 1800 miles; after another year or so, they may well have been stuck or cold-welded. Let other owners of titanium Motobecanes beware: I like the bike very much, and I'm not bashing BD in any respect, but this part of the assembly could have been better on my bike. Too, there was no wavy spring washer installed on either side of the spindle; if the assemblers had the washers, they did not install one on this bike.
The drive-train now operates as it should, more or less quietly.