The Mothers En Vogue Knit Shirred Breastfeeding Dress has hidden zippers that allow access for feeding and the skivvy (which only goes as low as the bust) is part of the garment.
This dress is so clever. You may not even be able to see the zips in the picture but there are hidden zips that undo to allow access for feeding and the black skivvy underneath is part of the garment and only long enough to just cover the bust.