Put simply, it would require a fair amount of work:
1: create a php file that includes all the necessary files to be able to select the right product (base it on customer/product.php) but instead of displaying home.tpl - get it to show a new template (base this on product.tpl)
2: rewrite the links for the upsell products to go to this page as a pop-up (will need some javascript in there too to open the pop-up and to name the original window so that the pop-up can target it when add to cart is clicked)
enjoy
