Short-term contract marriages come up in quite a lot of SF of a certain tradition, but they often don't get explored very well (or the romance storyline is all about how the permanent lifemate marriage is so much better than the previous short-term ones, which kind of makes me go O.o) I didn't list it on this post because it's really more serial monogamy than poly, but I suspect there are a lot of canons that would benefit from that!

The way Liaden deals with inheritance and family with that in the background sounds really interesting, though. You'd have to have something like that if you wanted inheritance to still work, and having a formal Line sounds like a good way to do it.