Is it possible to collect a VAT number and update the totals in an Apple Pay checkout experience?

Looking through the Apple Pay documentation, I see that it is possible to define the shipping contact fields like postal code and locality. I also see that there are callbacks when the shipping address changes so that sales tax and/or shipping methods can be changed. However, I can't find any means of adding a field for collecting VAT, which is required if one is to change the tax rate based on the customer's Vat Number. Does anyone know how to do this, or know that this is *explicitly* not supported?