How do I make /sdcard link point to external SD Card on Android 4.4.2?
As far as I know, you can change this by editing /system/etc/vold.fstab, but if it doesn't have what you need, you might have to tamper with init.rc inside boot.img.

I'm not sure if what you're aiming to do is safe though, what with the recent switch to fuse sdcards.