I've never used it but `man crypttab` has no option 'luks' listed as available to be used in the 4th field.
I would also suspect the permissions on the file containing the password will need to be fairly restrictive so it'll need to be chmod 600 and owned root:root. A read of /etc/init.d/functions in the init_crypto function confirms this. It also shows that as of CentOS 6, UUIDs are supported.
You do know that /etc/crypttab is only used during boot? If you change it then you need to reboot to have the changes reflected.