This is one of the 'features' of Payments Pro. When the user decides to checkout with PayPal most of the user's information is not passed along. PayPal will only ever send back the shipping address, it should be the one that is on file. The API that allows eCart to retrieve the details of the order will only pass the information for the shipping and not the billing, PayPal keeps this information private and hidden from the seller.
As far as I know you should be able to get back a custom value so long as it is passed through the entire checkout. How are you sending the value and where are you trying to display it after the fact?
The details for the order including the address2 value is held in the db. So while you might not see it on the checkout success page it has been recorded and is available in your db if you have recorded it. You can also add the address2 to the display on the checkout success page.
