I am working on the `Relationship` type in Activity Streams 2.0 for my book on ActivityPub, which took me into the FOAF vocabulary. I saw that there's a "geek code" property defined, which, like, wow.
http://xmlns.com/foaf/spec/#term_geekcode
You could reasonably add a 'foaf:geekcode' property to an ActivityPub actor, for example.