I have had great success with https://www.backlift.com/ . They support custom domains and have a few really helpful APIS (like one for persisting data).
If you're really excited about this concept, you can do it right now with paperplane.io, no waiting until October!
Full disclosure: I run paperplane.io :)
Do you have a feature list somewhere? One of the things that convinced me of signing up to the Brace.io waiting list is the "staging" site.
We don't currently support the "staging" site in the same way. It's easy enough to have multiple sites with similar content. Another feature that's currently in development and not listed anywhere is automatic compilation of haml/sass/markdown, etc.
This is really fantastic I always wondered if there was a way to do this.