Okay, stumbled into this little one-shot fic today. I wish it was longer or even abandoned WIP as this is basically just a premise of a longer story but I liked this a lot. It's HBP-compatible, rating NC-17/mature. It's written by Shiv5468 and it's called "Fantasies".
http://www.digital-quill.org//viewstory.php?sid=1028