Display Further Instructions

I'd like to be able to display a message for users upon entering a certain coupon to give further instructions.

Use Case: I would like to extend a discount to members of another site. I'd like to have the users, who will probably be purchasing products as anonymous users on my site at first, enter in their user name from the affiliate site into the order notes, so that I can verify that the coupon is valid for them to use.

This should be as simple as a hook_form_alter on the coupon edit page, and a function to dsm the message when the discount is applied to the order.