We have a number of items which use a discounted custom price for logged-in customers (part of an older promotion). We're looking at running a new promotion that will automatically discount prices for everyone. However we don't want the two promotions to be additive, we want the customer to get one discount or the other (the item's custom price, or the shopping cart rule)--whichever is greater.
How would I determine that a cart contains items with custom prices and also qualifies for a shopping cart rule?