emerge -av rpm
rpm -i --nodeps hll2310dpdrv-4.0.0-1.i386.rpm
...
lpadmin -p HLL2310D -E -v usb://dev/usb/lp0 -P /usr/share/ppd/brother/brother-HLL2310D-cups-en.ppd
lpadmin: Printer drivers are deprecated and will stop working in a future version of CUPS.