This looks really interesting. We've been using the nginx-push-stream-module to achieve something similar, but the public facing HTTP API design isn't very flexible, so something that lets you configure this explicitly would be great.
Thanks for the comment. At my last job, we were unable to migrate an existing API over to a certain realtime solution without breakage, and that was motivation to create something more versatile.
https://github.com/wandenberg/nginx-push-stream-module