In http://wiki.openwrt.org/doc/howto/extroot it says "Notably, in OpenWrt 10 (Backfire) you cannot use the pivot overlay variant, not pivot root." Which seams to say neither extroot will work. Perhaps it should be "Notably, in OpenWrt 10 (Backfire) you cannot use the pivot overlay variant, MUST USE pivot root."?
Could someone who knows this stuff, unlike myself, update this please?