@davidbisset If you set the HTTP header to Accept: application/atom+xml in your request, you get an ATOM feed. So like this: curl -H 'Accept: application/atom+xml' https://github.com/dimensionmedia?tab=stars. This works with many URLs on Github.
https://cactus.chat/, Cactus Comments is a federated comment system for the web, based on the Matrix protocol. Good idea but I am not sure why this needs an appservice for synapse. I think it could also be just done using a single room and storing the post metadata inside the chat message JSON.