Hmm... regarding the hook, you'll have to chat with Lyle about it. I'm not really sure of all the details. For your application, it seems like you may be needing to limit unique items in the cart? It seems like you'll want to not show the quantity box on that page and hook into the add to cart form to make sure an item w/ that e-mail doesn't already exist in the cart and then also hook into the cart update form to force the quantity to stay at 1. I think in the future we really need a way for the cart form to control whether or not to display a quantity box. For now you may just have to alter the form to force it to be 1 and make that a bit of text instead of a form textfield.