I've never been one of tape right on the skin, except in certain places.

But if you want to use your idea and cause physical discomfort at the same time, use clothespins.

Affix three or four clothespins to each labia and the tape the free ends to her thighs using either medical tape or duct tape. This opens her like you want, but also makes thing uncomfortable for her. It's also a littleb bit safer, in my opinion, than applying tape directly to mucous-producing areas of the body.

Added bonus: The clothespins and what type of tape you use also allow you to control exactly how open she is. By pulling the clothespins as widelt away from her body as you wish and then taping them down with stronger tape, like duct tape or packing tape, you can really stretch her.