Hi,
Using views, you can put some code in the "checkout / step" but the problem would be that, the customer should go to the checkout to have the product modification.
I think that the trigger "onBeforeCartUpdate" would be better.
This trigger have the parameters: &$cartClass, &$cart, $product_id, $quantity, $add, $type, $resetCartWhenUpdate, $force, &$do
So, by making a small plugin, you would be able to get the cart content before his update.
Checking products, removing or adding some.
The base of this trigger is when the $product_id would be update with the $quantity.
The parameter $do allows you to forbidden the update.
The $cart object would contain the current cart, before the update of the product ($product_id).
Regards,