Thanks Mark for the tip – unfortunately it was not as simple as you described. Client wanted the buy now link in #cboxBottomCenter and also did not want to have to build a project page for each item (he has thousands). In the end I rolled a little js to get the desired outcome:
jQuery(document).bind('cbox_complete', function(){
// get the title
var t = jQuery('#cboxTitle').text();
// split it
var arr = t.split(' ');
var imageId = arr[0].replace('#','');
jQuery('#cboxBottomCenter').append("<a class='cbox-add-to-cart' style='float:left' margin-top: -10px;' href='cart/?image="+imageId+"'>Purchase Print</a>");
});
Thankfully he includes the product id on all product titles. I wrote a custom page template that handles the incoming request and displays the product and cart stuff.