The concept is fine and everything, but I'd hate to HAVE to sleep in a down jacket every night on the AT. I prefer to keep my sleeping gear and outer clothes seperate for the most part. That's not to say that I don't sleep in my jackets and stuff to stretch out the lower range, but it worse comes to worse, I know my sleeping bag is going to be dry, whereas I can't 100% guarantee my jacket will be.
Granted, I almost always just use my jacket as a pillow, and my down gear hasn't been wet in 15 years, but you never know. Now that I'm slowly converting to the hammock thing, I might start ditching the jacket in favor of draping my underquilt over me or sewing a "head hole" slit ala a No Snivller.
If I thru-hiked again, I'd consider going that route, pairing that with my Marmot DriCime shirt. The DriClime's a little heavier than a windshirt/undershirt combination, but it's probably my most-used piece of gear overall.