You can always use mount –bind to do the magic…
# mount –bind dir1 dir2
This will make dir1 to be mounted “over” dir2 (thus making the original contents of dir2 – and all its subfolders – unaccessible until you unmount it, so, be careful)
Hope it is what you are looking for…
Note: You can, once confirming you accomplished what you intended, use the pre-boot scripts to make it persistent after each boot.