I have seen it but haven't looked into it too deeply. I think it's more complex/sophisticated than what I have in mind. i.e. it's meant to be a protocol for an entire social networking app, whereas I'm interested in a much smaller scope: just a raw event log. I figure if that's in place, everything else can be built on top. Heck, you could even use it as a base for an ActivityPub implementation! Write a tweet -> gets imported into your RSS event log -> gets published via ActivityPub.
Also, there is another protocol called Pubsubhubbub (yeah, I know...) which adds push on top of Atom/RSS. I know of at least one company, Superfeedr, that runs an implementation as a service (they were acquired by Medium): https://superfeedr.com/