I made my #plan9 / #9front based #ActivityPub repo public. It's still very much a work in progress and it's mostly just a hodgepodge of undocumented scripts and C but if anyone's interested in helping push it along it's at https://github.com/driusan/fedi9.git

Right now it can:
Client:
- get (public) outboxes and cache them in ndb db
- import following list and do same
- serve them over 9p

Server:
- handle webfinger requests
- cache incoming inbox posts
- validate httpsigs