Removing out of stock items from cart for customers with no terms

Hi. We need to allow back-ordering for customers with terms and not allow it for customers without terms. So what I thought I could do is use scriptable cart to check the customers terms and then remove out of stock items from the cart if they have no terms.

I'm running into permissions problems - I need to check the item records to get the quantities available per item and the web store users are assigned to the customer centre role which doesn't have the necessary permissions. I need to use a client script so I can't 'run it as admin'.