Magento - How do I use a shopping cart price rule to display a cms block?

[graffin]

I would like to use a shopping cart price rule to display a cross sell message rather than setting a discount.For instance, if a certain item is in the cart, display the promotional message in the cross sell area.Specifically, we have jacket and pants that can be purchased separately. But when purchased together, the customer can save $50. So, if the jacket is in the cart, I would like a message that says "buy the pants and save $50" and vice versa. There is already a rule in place to give the discount when both are in the cart, so I would like to call their attention to the discount.