You might want to ask Alan Runyan at Enfold Systems if he'd be willing to give you access to his Ploud code (Plone in the cloud).
In a way, anyone running more than a couple of Plone sites probably already has their own PaaS, though probably not as automated as you might need if you wanted to open it up to the public. e.g. any Plone installation lets you create as many Plone sites as you want, but then you have to add registration, billing, quotas, monitoring.
Plone.io would be PaaS as a headless CMS backend, plus a nice UI and content for helping devs create and deploy a modern JS frontend with it.