Feature request for checkout.js

[John]

When data-billing-address=true I would like to be able to populate the address fields, and allow the customer to edit the values.

In my case the customer has already filled out a form with their name and address, which I need to calculate shipping etc. If I want to validate their credit card billing address I have to require they type their billing address again in the pop-up. Having the pop-up fields pre-filled ready for editing, if needed, would be a more elegant solution.