Need repository for multiple site installs

I run a system that sets up common sites using a repository for code. So that I only have to change things in the repository for it to affect all of the sites. I use AG on these sites and it would be great if I could use the repository for it's base code. So that when there are upgrades, I don't have to manually update all 43 sites. (and growing) I have tried the base_url variable, but as I'm sure you know, that doesn't work with the includes and such.

Is there a possible Multi-Site configuration version in the making?
Or do you have any simple tips?
Or should I just dive in, figure it out for myself, then let ya know?
Thanks for the input.