At the moment, the result is "the only reasonable way for a member of the public to post to their Google+ profile is via the web-facing interface". Well, Sort of. OK, not really.
There's a ton of what looks like undocumented/exposed API surface for getting to Google+ through the OAuth2 interfaces, but it's reasonably well locked down with white-listing as per documentation. I can see and access posting calls, but all return 403 or 405. The Google+ Domains interface, naturally, only works on GSuite/GApp profiles. In short, after some light probing with a stick, it's all pretty much exactly as documented.
I can use the 'share' URLs, but those require an SID, which requires a browser and user interaction. Snagging and reusing an SID works, but they time out in six months. That's not really acceptable. Storing a Gmail username and password in cleartext on a remote server would be far far super-balls even worse.
And then I noticed that Buffer implements an OAuth2-facing programmatic post interface that happily forwards to Google+ without mangling anything. Their API is exactly what I wanted. And the documentation was even good. Like really good.
Well, foo. That kind of took the wind out of my sails. Someone did all the work for me, exactly like I'd have done it, after all.
So I actually get exactly what I wanted, even if it's slightly Rube Goldberg-y. No idea how long it will last, but I suspect that if things break it's more likely to be changes to Google+ than something at Buffer.