[cflsystems]

I believe the backorder module will only show the message if product is out of stock and set to be available for backorder. But you can modify this behavior.
However if you only want to show a message and not actually allow for backorder out of stock products it's probably best to just have a custom module to add message to each product and show it on the page. Just as informational field nothing else.
You can add this a an attribute and will show if not empty but will show under spects tab not where the quantity is.