Invoke /system/bin/sdcard manually via terminal
Are you putting this command in an init.*.rc file? My understanding is you would have this in a init.extmount.rc for example. This requires extracting and repacking the boot.img. 

Since I had nothing better to do, I created this: https://github.com/BenBaltz/mountext

Its a recovery flashable zip to install your script by unpacking the boot.img and repacking it with the modifications. 

EDIT
Terminal commands

su -c mount -o bind /mnt/ext_sdcard /path/to/new/dir