Hardcoded form

No hook, but you could make a grouped product and use a url with a query string:

For example, if I have two simple products with ids 1593 (iPad) and 1801 (iPhone), and my grouped product is id 3111 (my-bundle), then my add-to-cart query string is:

http://www.mysite.com/shop/cart/?add-to-cart=3111&quantity[1593]=5&quantity[1801]=2

The url may vary a bit depending on your permalink setting.

Use css to style the link like a button.