I described my connection poorly; I'm actually not using the wifi on the OPi. It has an ethernet connection to my router, and the router connects to the printer over wifi. So as far as the OPi is concerned, it's an ethernet connection.
Anyhoo! Yes, printing over USB works . I can't readily connect the OPi directly to the printer via ethernet, since OPi's ethernet is how it connects to the local network. But I did try connecting the printer to the router on ethernet (rather than wifi), and it works . And then when I took the printer off ethernet and back on wifi, leaving CUPS with the same configuration, it continues to work ! Only thing I can see that might be different is that, when connected on ethernet, I had CUPS use LPD instead of DNSSD. Don't remember if I had tried that previously. Even at that, I still had to manually edit /etc/cups/printers.conf to to put the printer's static IP in the device URI, in place of the "friendly" name that CUPS had selected. So this whole thing is probably 100% user error/inexperience. Thank you @Tido @usual user @martinayotte so much for your help and your patience with me!
mc510 got a reaction from Tido in Problem configuring CUPS