One of the larger difficulties for publishers is that
many of the 3rd party SSPs aren't ready to go full HTTPs
Right, but Google can motivate or improve the third-party advertisers to update much more effectively than publishers can - just Google hasn't chosen to do that yet.
It would be easy to proxy http-only ads from through a CDN that added encryption. Or to charge a premium to http-only ad networks, and ramp the premium up over time.
How do you think Google can motivate 3rd party advertisers? I'm talking about the Rubicons, Pubmatics, etc. Google doesn't have any real leverage over them. Other than the fact that as publishers do move to SSL (because of the SEO penalty that non SSL sites have) they won't use the SSPs that don't demand or can't insure 100% SSL for their buyers. So in effect Google is putting pressure on them to do it.
I don't understand your next part at all. Who would charge a premium to http only networks? Those sites don't actually rely on Google for delivery.
It would be easy to proxy http-only ads from through a CDN that added encryption. Or to charge a premium to http-only ad networks, and ramp the premium up over time.