I've finished creating this module. See the readme.txt for details. It will allow you to add addresses to a table from /user/'uid'. I would love to make a few minor changes to core to allow this module to be used during the order submit process.

Let me know what you think. There are a few minor //TODO's that could be completed. Most of which I don't know how to do yet. I based most of this off of uc_order and uc_cart. I don't know how to work with $_SESSION, so some of that might need to be added or removed.