Woah! just discovered that kmod-usb-net and kmod-usb-net-cdc-ether are available as kernel packages!
Just installed - plugged in my pocket beagle with usb otg - and woah, a new ethernet connection. Doesn't set the address yet you anything, but doing that by hand, and indeed my pocket beagle appears on the network.
Looks like my beagle farm may move from the NAS to the router!