My mechanic, Greg Hayman and his brother at Autofirme, in Amenia, NY just finished converting an RS6 sedan into an Avant for a customer in NYC. They replaced all the sheet metal from the A piller back, and it was seemless. You couldn't tell it from an original except for the lack of a roof rack. He told me that European Car will probably be doing an article on it. I saw a ton of pictures of the car under construction and will post some if I can.
Greg told me it was actually cheaper to convert it than to federalize an imported one.
Those of you who went on the first Northeast Ride will remember the shop. We stopped there and looked at some of the cars he had there.