Lets say you want to limit the backorder qty.
You create an extra product attribute where you store the maximum quantity allowed for backorders.
So if a product has
qty 10 and
backorder_limit 5 you want a customer to be able to add to the cart and buy 15 but showing the message:
"We don't have as many "xxxx" as you requested, but we'll back order the remaining 5."
If the customer try to add 1 or more to the cart you want to prevent this and show the message:
We don't have as many "xxxx" as you requested.
and prevent the extra qty to be added to the cart.