It's the only way I've been able to find. /sbin/reboot -f on a system with mounted filesystems does not trigger this problem. Reboot after unmounting does not trigger this problem. Reboot *during* a umount, and *boom* goodbye fs.
I have speculated on ways to fix this for good, though they require a new syscall, a new userspace utility, changes to shutdown scripts, that others on l-k agree my idea is not utterly insane, and for me to bother to implement all of this. The latter is questionable, given the number of things I mean to do that I never get around to. :)