How can I set up a discount that applies to all products in a cart when a promo code is used?

I'm trying to set up a discount that takes 10% off each product in the cart. Our CRM requires that the math be done this way.  I can't just take 10% off the subtotal because then I'd have a single value for the discount, not specific values for each product.  I've tried to write code to distribute the amount, but its always off by a penny (fractional cent rounding issues).

Maybe you can try using discount type "Assigned to Category".  Then create an unpublished category, and assign all products to it (best done in SQL ;)