Finally, things seem to start moving in the right direction:
Meet @forgefed, an ActivityPub / Federation protocol for networked version control services like GitHub or GitLab.
@fribbledom yeah, sure, maybe you have noticed their (specifically fr33domlover's) project called Vervis.
It is undoubtedly cool because (a) it's a demo for federated VCS hosts, and (b) it's written in Haskell, but it is also where the problem is.
(Cont.)
@fribbledom You see, GitLab CE, Phabricator, gogs and gitea are really good shit out there. So, obviously it would unrealistic to make one from scratch.
What if we instead fork one? Well, maybe, just make sure we have time maintaining all those forks. I sUrE hOpE sO!
What if we try to politely ask all those maintainers of gitea et al. to take federation into their roadmap? Allegedly it's hard to implement AP in Go bc there's a lot of Objects.
(Cont.)
@fribbledom so, finally here's my idea, that is, a backend application that's dedicated to AP server-to-server; the intra-server interactions and web are still managed by gitea et al. When an inter-server interaction needs to be sent/is received, the backend resolves it and, if required, manipulate the server's data via the platform's API, which is a workaround I thought would work. This way we would need much fewer changes to gitea et al.'s codebase. 
