But there is a usb backend that ships with Cups (found via `equery f cups | grep usb`) and it does find the device...

/usr/libexec/cups/backend/usb 
DEBUG: Loading USB quirks from "/usr/share/cups/usb".
DEBUG: Loaded 181 quirks.
DEBUG: list_devices
DEBUG: libusb_get_device_list=12
DEBUG2: Printer found with device ID: MFG:Brother;CMD:PJL,HBP;MDL:HL-L2310D series;CLS:PRINTER;CID:Brother Laser Type1; Device URI: usb://Brother/HL-L2310D%20series?serial=E78096L9N400161
direct usb://Brother/HL-L2310D%20series?serial=E78096L9N400161 "Brother HL-L2310D series" "Brother HL-L2310D series" "MFG:Brother;CMD:PJL,HBP;MDL:HL-L2310D series;CLS:PRINTER;CID:Brother Laser Type1;" ""