I think what confused me, and what you and nostr:npub12262qa4uhw7u8gdwlgmntqtv7aye8vdcmvszkqwgs0zchel6mz7s6cgrkj just clarified, is that there are actually two different things needed (a unique d-tag, so that the rating is replaceable), and a way to refer to something (so that the client can figure out what is being rated), and that these two things don't have to be within one tag.

You can use the normal i, a, t, whatever indexes for the second, and use any d-tag structure you want, for the first.