This is another great example of the add-to-cart without a full product node function I have been trying to develop. What you could do create a product node and then "plug in" the necessary theme_uc_product_add_to_cart pieces. I am taking that function name off the top of my head btw... but it something like that. Anyway, several of us are now looking to get this workings and I think it would solve your situation as well.

This point is being discussed in another thread as well:

http://www.ubercart.org/forum/support/512/how_i_setup_direct_add_cart_li...

Steve

p.s. Actually a few threads have explored aspects of this:
http://www.ubercart.org/forum/support/360/possible_add_products_cart_tex...
http://www.ubercart.org/forum/support/507/thickbox_add_cart
http://www.ubercart.org/forum/support/412/inject_product_static_page