If we agree to extend ActivityPub for git federation, we'd need to specify the context on how we'd add vocabs to the ActivityStream.

The ActivityPub community group is drafting an extension process.
Things are still under discussion and we'll have to wait for the finalized version.
https://github.com/w3c/activitystreams/wiki/Extension-process

Based on the current draft, this is probably what we have to do in the development process:

Multiple-implementation extensions still in development

For when we're experimenting with new problem areas that need new vocabulary terms, but we're not quite sure about them yet. For these, we should:

• Declare it with a Github issue

• use some kind of sub-namespace of AS2 (https://www.w3.org/ns/activitystreams/dating# or https://www.w3.org/ns/activitystreams#dating?)

• Implementors use the namespace in their contexts and the namespaced values in their documents:

   {
       "@context": ["https://www.w3.org/ns/activitystreams/",
       {"dating": "https://www.w3.org/ns/activitystreams/dating#"}],
       "type": "Person",
       "dating:seekingGender": "any",
       "dating:geographicalRestrictions": {
           "dating:within": "50km",
           "dating:of": "https://places.example/New_York_City"
       }
   }