In Slackware 11, usbfs is mounted from rc.udev with
mount usbfs -t usbfs /proc/bus/usb -o devgid=10,devmode=0666
In Slackware 12, usbfs is mounted from rc.S with
/sbin/mount -v usbfs /proc/bus/usb -t usbfs
Which means that only root is able to rw in usbfs.
You must edit rc.S to mount usbfs with rw permissions for other users (I used `wheel’ group):
/sbin/mount -v usbfs /proc/bus/usb -t usbfs -o devgid=10,devmode=0664