I was able to nfs mount a filesystem on my target over usbnet (.config attached).
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I can see the Ethernet gadget in the kernel boot messages.
[ 1.290649] gadget: using random self ethernet address
[ 1.296112] gadget: using random host ethernet address
[ 1.302062] usb0: MAC 2e:04:c5:66:fe:c7
[ 1.306091] usb0: HOST MAC f6:fb:d4:8b:e2:7a
[ 1.310607] gadget: Ethernet Gadget, version: Memorial Day 2008
[ 1.316894] gadget: g_ether ready
Once I get to the command line I manually bring up the usb0.
# ifconfig usb0 up
Start the dhcp client on usb0 interface
# udhcpc -i usb0
Mount the FS
# mount -o nolock <usb0-ipaddr>:/<host-dir> <local-dir>