Funny you should ask. I'm building one right now. It'll be available next month. Basically it's what you'd expect from a SSG from a code standpoint, but deploys with a live CMS so that non-literate users can edit it.
Looking forward to seeing webhook in action (I backed it). Will there be an open source component (perhaps the way Harp works), or entirely your service? Either way, it looks fantastic!
(I write code, so it's not for me, but I am wondering if I could say, setup a template for a friend, and have them fill in the content of the pages.)