Take a look at this link:http://www.tothewoods.net/StuffingDown.html
JustJeff shows a way to put down into the quilt so I think you could use a similar means to extract it from the underside of the bag.
Part of the issue is sewing up the cuts or seams that needed to be opened in order to remove the down.
When you are making the quilt, in the first place, you haven't cut the nylon or undone a seam where you are inserting the down.
To me, that is the biggest problem in doing this. Making sure that after you are done, down doesn't leak out and the bag isn't unduly weakened.