Giles, what your your fstab look like on the beaglebone black ? Also I’m not 100% sure about this but for consistancy you should not use mmcargs, but instead use netargs
This is what my own netargs line looks like:
netargs=setenv bootargs console=${console} ${optargs} root=/dev/nfs nfsroot=${serverip}:${rootpath},vers=3 rw ip=${ipaddr}
NFS version 3 works fine on Debian, perhaps Ubuntu is different ? I know you’ve read other guides but perhaps reading my guild to double check what you’ve done is complete. My guide: http://www.embeddedhobbyist.com/debian-tips/beaglebone-black/beaglebone-black-nfs-root/
scroll down to Configuring the NFS server and read down to see if you perhaps missed something. Also keep in mind that Although Ubuntu is based on Debian, package names can be different, and not everything is done the same way.