To be honest, and speaking purely unofficially, if I were going to tag my own fic, I would only use "polyamory" on fics about modern people who self-identify as poly.

I think, ultimately, this is what I've settled on (with the caveat that they wouldn't necessarily use the word "poly" in so identifying, so I might be a little broader than you--I haven't really had to think about it wrt aliens or ancient Romans). I feel a little uncomfortable about this, because if there's not a sentence in the fic about the character identifying as poly then I sorta feel as if I'm violating the Death of the Author, but so it goes. And just because a character who in my head is poly appears in a fic probably still isn't enough to mark it as poly; they'd at the very least have to have sex or date somebody or something.