Add code to modal checkout popup

Hi,

I would like to add some code to the payment form / coupon entry area that pops up when you go to pay for an ad (classifieds). Basically I have a plugin that allows customers to like or tweet etc. your site to get a coupon code that they could then enter into the final payment popup. I would need to add a bit of php and html that provides a link. Is there anyway to hook into this modal form so I can add it to my child functions file?

Cheers!
Chris