That... may not work. I'm seeing corruption on journal replay of any ext4 filesystem mounted read-write in 3.6.3 and then uncleanly unmounted (for the full deeply sordid story of why basically random fses on my system get unmounted uncleanly on a clean shutdown, see my latest LKML post to the relevant thread: mea culpa for never revisiting the ugly shutdown script hack in question in many years, but I'd honestly forgotten it was there).
I suspect /usr/src survived simply because, in the 3.6.3 session when I wrote to it, it happened to get cleanly unmounted rather than uncleanly. (Why /var gets hit so much more often than other dirty filesystems, I still am not quite clear on. I suspect something using /var is not dying when it should when I shut my system down, holding /var open so it never gets unmounted properly and the bug always hits it.)