Wholesale promotion - discount 1.00 off per case

In the wholesale/distribution business, it is very (very) common to offer a dollar amount discount per case sold of a particular item.  Does anybody know how to do this using Discount Items? 

I know that the dollar amount could be reduced simply by overriding the price on the sales order, but we don't want to do it that way because there's no way to track the discount.  We need it to be a discount item so it hits the appropriate GL account (Sales Discounts).

We'd also like to be able to offer a dollar amount off per case at the order level using a promotion, and its related discount item, so that it applies to the total quantity (in cases - or other units of measure) on the order.  But frankly I'd be happy to get it at the line item level (vs order level) at this point.