Display upsell promotions on cart page

The promotion

In the marketing system a promotion is created:

Buy for more than $500, and get Product-A with a 50% discount

The promotion works just fine when Product-A is in the cart and the order total is > $500

But if the order total is less than $500 we want to inform the customer that he is missing out on a great discount

How do I implement this feature?

I see at least a few options when loading the cart page

1. An a ContentReference property to the CartPage to allow the editor to choose what promotion to display. But how can a editor select a promotion?
2. Loop through all active campaigns and find the first order level promotion and hope that this is the correct one
3. Extend the promotion with a checkbox "bool ShowOnBasketPage" - makes it easier to determine what promotion to show on the cart page

Any recommendations?