First, welcome to the community of people working on this important problem. I highly recommend you join this group to stay informed about what's going on elsewhere:
It's an "issue" group, not dedicated to any one protocol, service, or software package. Please, make sure you're a part of it!
A couple of notes:
1. We're working on including private messages in PubSubHubbub 0.4 and thus into the next version of OStatus. Understood that it's a big deal.
2. You're right, there's no standard API. ActivityPub is an attempt at that; see here: http://www.w3.org/community/activitypub/
Thanks for considering. Let's make sure we interoperate!