Finally found Carsten's answer (1) to this same question from four years ago. Don't know why I missed it while searching the archives yesterday.

Still, I've fumbled around for a while trying to understand how to get the org-id module turned on, and how to write the syntax in the "remote()" formula. (No syntax at all, it turns out. Just the ID value itself.)

(1) https://www.mail-archive.com/emacs-orgmode@gnu.org/msg21313.html