How to access values from earlier steps in multi-page Webform?
This does not answer the question in your title, but reading your use case it sounds like the scenario that Profile2 was written for.  Related stackoverflow post

Webform is great for surveys and such, but when you want "to be able to use them as default values for future fields" they need to be connected to the user.  So you would then need more custom code to fetch the user's historic webform submission to pull these values in a form alter for your billing address.  Profile(2) is written precisely to make this easier.

Also, the less last-minute custom coding you are doing before handing this off to some one else, the less that some one else will curse you in their heart when they are trying to make updates.