Well, it turns out somebody (probably named dmo?) did all the hard work: Multiple branches using git-svn
If you’re like me, you need to track multiple branches in the same Git repository. I’m going to use this extensively in the XLsuite repository. I have 2 trunks/mainlines: one for the closed source, and one for the open sourced version, I also have a stable version, and some work in progress branches.