Report that shows open sales orders by allocated/not allocated

The advanced sales report functionality allows you to choose whether or not to allocate stock when the SO is created.  We have situations where we don't want to allocate stock immediately as we don't want to commit stock that is on hand straightaway when we know that a bunch of stock will be arriving later.  But this functionality is pretty useless if we cannot report on how much stock is on open SO's that is not allocated.  We need this information in order to be able to calculate if we need to order additional stock.  Thank you.