Actually, I’ve gone and done a little bit of reading around and it looks like using Series this way may not even really be endorsed… It seems like a bit of a grey area to me, especially given that there’s no better alternative, and a lot of editors are already making good use of Series in this sort of de-facto way, regardless of style leaders’ intents, ssssoooooooooo… ¯\_(ツ)_/¯
But, this does mean that a ticket for adding a relationship type between Series entities is likely to get shot down or ignored, given that we want it so that we can Use The Thing Wrong, even more. Now with 100% more Wrong!
(Edit: FWIW, I personally completely approve of this ‘run-away’ unintentional application. I prefer descriptivism over prescriptivism (any sane linguist will say the same thing), so if people “break the rules” doing something that makes sense, then IMO the rules were flawed in the first place. Using Series in this way is incredibly helpful, and incredibly healthy for the database, so style intent can go get dunked on this one.)