I ended up sticking my head into the discord for a nosy, and it looks like the devs probably won't be adding this (there's an item in the FAQ about how it's not possible for cats from different saves to interact), which is understandable since it'd really increase the scope of the project, so it'll have to stay in my imagination for now :') There is a mod called OtherClansGen that seems to do something like this, though!
I did have another thing I forgot to say, which was that I'd love if there was a hunger/freshkill-pile-fullness mechanic, so that hunting patrols would feel meaningful and leaf-fall and leaf-bare actually felt harsher. Maybe hungry cats would fail patrols or fall ill more often, and hungry apprentices would have a harder time gaining experience, that sort of thing :0
And thank you! That's good to know :D