For a network printer hooked to a macOS machine's network, you should be able to turn on print sharing, and share it with Mac OS 9, even if it doesn't have OS 9 drivers.
This will require you to setup a TAP network with Qemu, as SLIP networking won't be able to see anything on your network, and you need to be on the same net/subnet as the macOS print server.
On the Panther Mac that the printer is physically connected to, you need to find out the printer's CUPS name. You can do this by typing lpstat -v in Terminal, or through the CUPS web interface. If you use lpstat -v, the name is the first part of the line listing the printer. For me it was HP_LaserJet_6P.
Use the Desktop Printer Utility on the OS 9 Mac to setup an lpr printer. Use the "Generic" PPD. The "Printer Address" is the IP address of your server, and the "Queue" is your USB printer's name.
