All the cart checkout pages including the product details page are user controls of the buynow module so you will not be able to add other dnn modules to the product details page. In order for you to add 3rd party modules to the product details page, we would need to make the product details page its own module. If we did that then you would have to link the product list and cart pages with the product details page using a setting similar to what we have implemented in the featured products module. This would make the setup of the cart a bit more cumbersome and sacrifice some of the user friendliness of the cart which has been our focus. I can definitly see the reason why you would want to add 3rd party modules to the product details page and we will discuss this internally for future product enhancements. In the meantime you can add custom text, graphics and links to the product details page by modifying the productdetails.ascx page directly.
-Scott