Hi Anthrax-VX, I added a comment on your post via the Plugin page. I’m getting an AJAX developer in to add the auto-population of Checkout fields based on the User dropdown. This will pull stored Checkout details if available and default to the WordPress default User details (e.g. First Name, Last Name, e-mail address, etc.).
I agree this is a better solution to simply disabling the fields as it will allow you to override Checkout details if they are out of date or the customer simply wants their Sale delivered to another address for example. I hope to have this cooked in this fortnight.