If you have a client proxy on a server, then the phone client can remain just as Damus already is. But,
1) There has to be a trust relationship between that phone client and that relay proxy, and in real world situations, nobody is managing their own client proxy server and nobody wants to trust another service if they have the option to not trust anybody by using an outbox model client, and
2) The client proxy has to do the outbox model for them, not be a dumb relay, or else the user experience is broken and people can't follow people anywhere, they are stuck with the limited community that you provide them.