I was pointed to this proposal for fedi: from Tim Bray, which I’d missed, and found a commenter pointing back to this advocating for acct:, which leaves the question of linking to posts kinda vague

EDIT: adding FEP-07d7: A Custom URL Scheme and Web-Based Protocol Handlers for Linking to ActivityPub Resources ( discussion )

There are at least a couple of Riverworld attempts. Haven’t seen either

https://en.wikipedia.org/wiki/Riverworld_(2003_film) https://en.wikipedia.org/wiki/Riverworld_(2010_miniseries)