In case you were racked with suspense, returning the shorts with the lip paintings went just fine. I now have a new piece ready for return: a black sundress from Gap.
I chose the sundress because although it had a nice substantial underskirt, the top layer was not at all diaphanous, making the space between the two layers relatively secure for my purposes. I sewed a zipper to the top side of the underskirt with a little fold of fabric in between gripping sides, to which I then added a strip of bright white lace. Here's a shot with the skirt hiked up so that you can see the underskirt, and the added zipper, in its zipped state:
Here are some shots of the zipper opening, peep-show style:
And another shot, to give you a sense of overall placement and scale:
Visually, the addition is nearly imperceptible, since nothing shows on the outside, unless you pull up the skirt (and even then it is black zipper on black fabric), and when turned entirely inside out, you would only see the black stitching used to hold on the zipper and lace, which is again highly unnoticeable against the black fabric. I am, though, less certain about how obvious the addition is in tactile terms. Thankfully, the fabric is rather stiff, so it is less likely that odd bunching, or a little more weight on one side than the other would give me away.