I'm pretty sure you can't even post to G+ using the API. In my opinion that's not even close to useful.
And you're pretty wrong. You do that by posting App Activities via the Moments API (which the user controls the visibility of using Circles, just like any other posts.)
https://developers.google.com/+/api/moment-types/ These come in a number of different types with different semantic properties, with a generic fallback type, and a publicly posted process to request new types.