There’s a weird experiment of sorts happening over on Write.as which CJ Eller calls “silent mentions”, but I’m not entire sure I get it. As near as I can tell, it’s just people posting to the same hashtag. It might not be a reply, as such, at all? There’s no new functionality here, per se, it doesn’t look like; it seems to be a user-generated social-hack of designating a hashtag on your own post and hoping no one else uses it for anything else?
ETA: Eller’s rapid response: “But sometimes I wonder if platform limitations are where interesting possibilities can emerge.”