I wasn't entirely aware of it but I've been waiting for someone to write a story about this!

As a thought - this one would require some forbidden surgery: She could get a hole drilled in the top and bottom of her skull via a trepanning drill (perhaps the bottom done laparoscopically from inside her mouth) and, with the help of a live x-ray feed, insert a pole that goes through her head so she doesn't have to hold her head back and her mouth open constantly. With any luck skin would gradually grow along the inside of the opening until she effectively has a vagina running the length of her body.