She shared the sketch and some in progress pics at DeviantArt, and I'm in love. Casey covered all the details. The black dress has silver seams to represent the car's body lines, the Chevrolet logo is added in silver embroidery, and the garter will have a holster for a flask of holy water or a colt or Ruby's demon knife (so many weapon options!). The license plate number will be painted on the back shoulder.
Creative as all of that is, the pièce de résistance of the Impala dress is the top. It's a fiberglass cast of the front of the car, scaled to fit the dress. Seriously. It has the grill and the headlights. Casey sculpted it from oil clay, made a mold from silicone, and cast the piece in fiberglass. Sounds like she's resourceful enough to be a hunter.
See photos from the entire process at DeviantArt.