Update: after moving /usr/local to /home/local I ended up building and loading a bunch of stuff -- including Postgres there. Then I updated systemd so it would autostart the database server on reboot. But once I did an actual reboot it ended up looping endlessly while trying to mount a filesystem. Couldn't break out of it (and haven't been able to figure out how to boot into single-user mode yet :-)
This is pure conjecture but my guess is at a certain point in startup /home isn't mounted yet but something needs to access a command in /usr/local. So we end up hanging. I ended up re-flashing and starting from scratch. This time I'm going to try to install non-system apps into /home/local and to make sure systemd has mounted all filesystems before trying to access any third-party binaries.
Anyway, thought I'd do a follow-up in case someone sees my previous note and tries doing it that way. Caveat emptor.