I can’t help but think about ActivityPub and how it could be used to implement something like this
Heard about ActivityPub the first time, just checked it out briefly. Ok it' some social networking protocol, how does it relate to write.as?
You can see and comment on write.as posts from other ActivityPub (AP) platforms that support it like Mastodon. It's up to the platform to implement it since the AP specification leaves implementation details mostly up to the implementor.