It's definitely a very rough work in progress but I have used it successfully to deploy my own simplenote server that works with all the clients.
Simperium Server: https://github.com/beaucollins/node-simperium-server
It's in the proof-of-concept stage so it doesn't persist the data on the server.
Protocol for reference: https://github.com/Simperium/simperium-protocol/blob/master/...