Currently, only RSC instance can properly federate natively, this is happening without re-inventing the wheel, the only thing another instance has to do, is to add an "all users rss feed" from the whole instance in the "approved federation" tab of the RSC admin interface, this is bi-directional, so it means, each instance has to add the subscriptions.
So for example, rsc.rmdes.be added bob.rmdes.be and alice.rmdes.be and bob and alice added rsc.rmdes.be, alice added bob and bob added alice, once this is done, any users from any of these 3 instance can talk to each other, with just RSS feeds.