If you don't mind losing one inch of length, you can form a half-inch self-casing at the waist. Flip the skirt portion inside out so it's over the bodice portion and iron a crease right at the waist (or exactly a half-inch below where you want the elastic), that is, if there isn't a waistline seam. In either case, sew along the waistline a half-inch from the crease/seam. Unpick one of the side seams ONLY in the new casing you made, and thread your elastic in with a safety pin. Stitch the two ends together, pull the garment so that they get sucked into the casing, and then whipstitch the seam-hole closed again.
This is so informative! Thank you. I think this will work out wonderfully. I'll be sure to update the thread when I finish the first dress.