Basically, the assumption is that you have your SRV records set up properly. Modern XMPP clients, like Dino, will then reconnect automatically to a server that they can access, and read history to find anything they missed. Horizontally scaling XMPP servers exist, it's probably up to you to find one that works well in a multi-site configuration though.
Your other option is just to give everyone their own XMPP address on a subdomain for each site - foo@dublin.example.com, bar@brussels.example.com, etc. I do this with a number of independent XMPP servers and it works fine. Then you're just depending on ordinary XMPP s2s to communicate between them, which is obviously well-supported.
