I have tried to complete this mod myself but it has thus far eluded me. On my product page I have a base product with several potential addons in the form of product variants. These addons can be selected with checkboxes, radio buttons and dropdowns depending on the characteristics of the add-on.
The problem I am having is with dropdowns. I want to add the text PLEASE SELECT to a dropdown with a NULL value for product variants however I keep getting the "Cannot find variant" error when the item is added to cart and a variant is not selected. It is my understanding that there is no null value that can be assigned to the <option> that \cart\add will accept.
My temporary solution to this problem has been to create $0 standard options which get added to the cart along with the base product if a dropdown selection has not been made. This is very clunky. Check it out for yourself here http://bit.ly/dkRVcU
Let me know if you are up for this quick challenge.