"I ran a large cluster where /usr was mounted read-only by all the compute nodes,
which PXE-booted with a bare-minimal / in initrd (just enough to mount /usr). Worked well most of the time except for shutdown, when the initscripts just couldn't handle the idea that /usr would go away at unmount-all time. I suppose this proposal wouldn't necessarily make things worse, but it will make it harder to populate an initrd with a minimal / setup if no one is even pretending to try to have a version of mount that can live w/o /usr anymore. I guess you'd have to make it out of busybox or something instead of just copying over selected files from /bin, /sbin, and /lib."
Exactly, this is essentially describing a modern initramfs. Nowadays, there's no need to mount /usr over the network in a cluster environment--you just NFS mount the rootfs (which can include /usr), making the need for a separate /usr in this context redundant.