Next Step...

That seemed cracked at first sight, but the more I think about it the more sense it makes. (I think SysV did something like it, too, but I don't have a SysV system nearby to check.)

It requires only one change to early boot: the initramfs needs to find and mount /usr as well as /. Since it's already finding /, this is unlikely to be remotely problematic: it can figure out where to find it by consulting the real /etc/fstab on the / it just mounted, as well.

I think I'll rejig my system this way and see what breaks.