Can you have multiple inboxes? The spec says something about allowing extensions — could you augment it to add your own extra inbox that anyone using your extended protocol would be able to access?
Best way I can think of right now is to deliver the updates to the recipients' inboxes, then have the recipients determine whether to show in the regular timeline or the extra one based on whether the sender is followed. But that would clutter the default timeline for other #ActivityPub clients.
smallcircles