Git submodule for a local branch?

I have a bunch of branches in my repo (a, b, c, …), and I'd like to check them out into subdirectories of another branch (index). My initial inclination was to use something like

  $ git checkout index
  $ git branch
    a
    b
    c
  * index
  $ git submodule add -b a --reference ./ ./ dir-for-a/
  $ git submodule add -b b --reference ./ ./ dir-for-b/
  $ git submodule add -b c --reference ./ ./ dir-for-c/

but cloning a remote repository (vs. checking out a local branch) seems to be baked into the submodule implementation. Should I be thinking about generalizing git-submodule.sh, or am I looking under the wrong rock? My ideal syntax would be something like

  $ git submodule add -b c --local dir-for-c/

The motivation is that I have website that contains a bunch of sub-sites, and the sub-sites share content. I have per-sub-site branches (a, b, c) and want a master branch (index) that aggregates them. Perhaps this is too much to wedge into a single repository?

Cheers, Trevor