You could create a collection for discountable items that includes all products except the non discountable ones. When you create the discount codes set them to apply to the discountables collection only.
It looks like in between me posting this and now, they updated it so you can use discount codes on the POS. This fixed my problem for me, as I just made a discount code that worked on everything but the bags. I can even make a button for each discount on the smart grid.