Since you have only one repo, you cannot use the "submodule following a branch" feature.
That leaves only symlinks (I agree here with Zach Latta's comment), even though they are tricky to put in place when used with Windows: see "Git Symlinks in Windows".
A symlink isn't exactly the equivalent of an SVN:external (since it cannot reference a specific revision like svn:external can), but it can be a good enough workaround in your case.