You can just use SVN, why not? It's a matter of a simple script to checkout the parent repo (main WP), and merge the changes into your own branch.
It doesn't seem that simple -- I'd have to track ongoing diffs to the main WP and apply those diffs to my local repository, with some failure mechanism when there was a conflict (where a conflict is any unappliable diff). Doable, but I'd rather not work out the details myself. Though I suppose there's probably scripts out there to do this already.# Ian Bicking