I see the from AJAX API that is possible to update the cart attributes. However I don't see a list of attributes that can be updated?
If it is possible to update the cart via the AJAX API, why cannot I see any reference to updating the cart in the standard API?
Perhaps set quantity to 0, add a new differently priced item?
Although this may work, creating a dynamic product on the fly based on an existing product is a hack. I'm going to look into it in the short term though. Thanks..