Aha, this is exactly the kind of thing I was looking for! Thank you!

Is it possible to do this preemptively as a not-logged-in user on another instance? Or does it have to be a member of the instance?

It seems like we’re going to end up with a bunch of copies of the same community if you have to know that a community exists on instance X in order to propogate it to your instance. I just did that with Linguistics.