I built a small experiment on top of the rss.chat API: https://chatouk.sofab.ch — an independent public-posts app that bridges with the rss.chat network.
It works both ways. A reply written in Chatouk is crossposted through the API and threads correctly here. And replies posted here flow back in real time into Chatouk threads via the WebSocket firehose, shown with their origin.
Nothing but open standards: RSS out, the rss.chat API for crossposting, the firehose for listening. The bridge setup is multi-server — this instance and https://rsschat.sofab.ch are connected so far, adding another takes one command.