A link like [this](/c/[email protected]) will work for website users on instances with at least one person subscribed to that community. There is an open issues to generalize this so that clicking the link doesn't crash Jerboa, and another one to load data from the remote instance instead of just 404ing. I know that's not amazing news, but this is an open source project under active development.
Posts cannot be universally linked across instances as each instance has a different id, correct. I'm not sure anybody's thought of a way around that, even before considering dev time. Definitely a longer-term issue to tackle.