Can I show a warning that the item is "Temporarily Out of Stock" to the users when the back order is allowed and inventory is out of stock?

 It is not possible to display any warning or note that says "temporarily out of stock" when the back order is allowed. When the back order is allowed the system will allow the user to place the order like a general product with enough inventory. However you can add this information in the name of Product and add some description to the long description of the product to say that the product is "temporarily out of stock" this will allow the users to see that the product is out of Stock when they are selecting the product and the long description will let them know any additional information.