The director suggested I put a zipper down the back and add fabric there, so I went with that.
The cut down the back of the dress and widening of the waist created a diamond-shaped opening (below left). I knew the inserted fabric following this shape would emphasize the waistline. So I inverted the diamond, making the lower part wide enough to cover the waist expansion, with the upper part emphasizing width in the upper back instead, with the triangles somewhat echoing the triangle design on the front of the dress (right).
Below you can see the original opening. On the right, the dotted lines show the raw edges of the dress underneath the insert.
Below I have placed the insert on top of the back of the dress, then pinned and top-stitched it on. Note the zipper was put in the insert before it was placed on the dress. Easier that way!
The dress originally had a side zipper. I removed the side zipper and closed the seam.
Here are the front and back of the costume, pressed and ready to go. The insert, cut on the bias, gave just the amount of stretch needed to fit the actress and give her room to move while retaining the form-fitted look.