Shopify Plus Scripts for checkout page - order notes

In Spain and Norway when you make a purchase that is to be imported you must provide your government issued ID number (something to do with tax).
I have built a simple JS script on the cart page. My script detects the users IP address and disables the ‘proceed to checkout’ button until they input a valid ID number (runs through a node module package for validation). This number is then sent as notes with the order. This works fine, but the underlying logic is flawed. The ID number is required when the items are being shipped to that country, but not necessarily where the items are being purchased from. Instead of looking for the users location, ideally we would base this upon the delivery destination input on the checkout - shipping address stage.

My question is this:
Can I write a script for the checkout page to do this? I can see from the docs that we can query the shipping address country, but I don’t see a way of adding additional notes to the order at this stage. Or even how to disable the purchase if a valid number is not provided.

Thanks in advance!