Hello Robbie,
after I backed up with this latest method I rebooted and found my n900 completely messed up. This happened also last time I did it, with your previous version (the tar one). To be clear, I didn't try any reboot.
Now I'm restoring with your first version, the img one, which works for me by flashing the rootfs and restoring the optfs.
Do you have any hint why the simple action of backup in some way damages the filesystem? This sounds me quite strange, but it's what is happening to me.