Yeah, in actual fact this is going to be trickier than i though..
the default buttons preform a form submit. it's not just the trigger that is needed, it is all the other post information from the form.
However, that will be a little tricky with the cart display page since each of the buttons tigers a different section of code, so it's not a generic submit but a submit triggered from a specific button.
I think you would be better of using the standard submit buttons or image form elements. you can use CSS to style them if you like.