Yes, but I think I was imagining a world without servers. For example, you could update your IP address (for people to contact you at) by sending a signed mess

Yes, but I think I was imagining a world without servers. For example, you could update your IP address (for people to contact you at) by sending a signed message to a global DHT, or, for more privacy, put the address of your current rendezvous server in the DHT instead. The rendezvous server would be responsible for forwarding incoming connections to you, which you could refuse based on the public key of the initiator. If the rendezvous server only stores a lookup between your public key and IP address, perhaps only in memory, for a few hours, before you switch to a different rendezvous server, that doesn't feel like having an "account" on a server.