I've found some additional details about this problem. First, if I stop
the CUPS service before I turn on the printer the probe works fine, and
locates the printer as a usblp device. I can then start CUPS and print
without issue (I assume... I'm not actually wasting paper testing this,
since the failure to discover the printer is the primary issue).

This, and some additional testing, suggest that the problem is related
to the timing of the probe. The printers in this line lack the internal
memory to store their firmware, and instead load the firmware when they
are first turned on or connected. It's obvious to the user when this is
happening because the printer noisily "runs" while the firmware loads
(the paper rollers turn, I guess). This takes a good 5 or 6 seconds.

I'm hypothesizing that probing the printer while the firmware is loading
causes the problem. Maybe it even corrupts the firmware.

Further investigation pending.

-- 
hybrid usb backend doesn't work with usblp
https://bugs.launchpad.net/bugs/543177
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