Mounting an NFS filesystem requires the loopback interface to be up
So, I decided to mount an NFS partition instead, but most of the commands I tried after running rpcbind (the portmapper) hung until they timed out. What I finally worked out is that the loopback interface has to be up ("ifconfig lo up") for them to work.
If I was trying to mount an NFS partition as my root partition, I would probably take the approach I used for the x86 live CD, which was to have a compressed minimal root filesystem as part of the kernel image (look into CONFIG_BLK_DEV_INITRD, CONFIG_INITRAMFS_SOURCE, etc.), which was able to bring up the appropriate interfaces (reading environment variables set in the uEnv.txt file for the NFS server, etc.).