@evan
That is exactly what I'm trying to work on, but haven't figured out how to do.
The combination of Ids being dereferencable and the vocabulary extensible is tough. I'd prefer the generic AP server hosting the activities (easy) and objects (hard) but since everything is extensible and something other than Update activities might mutate the object (replies for instance), we can't know how to keep up-to-date representations of objects, so we need to push that responsibility out to the apps.