Ref.: https://groups.google.com/d/msg/joomla-dev-general/bK4r4m3VEBg/i9N8J5hNGkcJ
...start simple, though it would take a bit of time to set up. Thinking out loud too, I'd take these steps:
- Strip down a "Joomla Core" version to the bare minimum (Platform, Content, Menus, Users, etc) and put that in a new github repo.
- Create Phing (or Hudson) scripts to create the "build" and delta's (just clean up what we already have for the full CMS - just put them in a repo too some people know what happens).