That's really interesting, though I don't think that private 1:1 messages are what most people have in mind when they think of interaction between content creators and their audience.

I think the simplest way of adding a more traditional "replies" feature to RSS would be to have some way of indicating an RSS <item> is a reply to another RSS item in a different feed. Then maybe add some sort of POST endpoint where anyone can request that a reply be linked back to from the author's post, and there you go: a fully decentralized comment system.