USB printers must attach as a ugen(4) device which means you need to
disable ulpt in your kernel - see config(8).
Since HPLIP communication with the printer is done via CUPS so you need
to give the _cups user r/w access to /dev/{usb,ugen}*

To make the hpaio backend available to SANE, simply add "hpaio" to
its configuration file.  As root:
    echo "hpaio" >> ${SYSCONFDIR}/sane.d/dll.conf