O...kay. I'm commenting here rather than on the fic itself.
First, the good: I really like your use of synonyms. Adds a lot to the story. You're definitely one of the better writers in this genre we've seen in a while, which is why I'm commenting.
The bad: First, while I like the whole anal aspect, I don't really see what limiting yourself to one hole out of three adds to the story. Most sex stories of this kind have a rising action wherein the victim is gradually introduced, pleasurably or otherwise, to different varieties of sex, with anal coming near the end. (I count three different double entendres in that sentence...) Jumping in at the end seems a waste of potential. You could easily have stretched Willow's section over half a dozen chapters, each focusing on a different taboo (exhibitionism, defloration, degradation, cum play and so on) before reaching the final humiliation of being gang-banged up the ass in public.
Speaking of which, I hope this isn't the last we'll see of Willow. She's hot and she's shy, which makes her perfect for this kind of story.
Now, some thematic points:
I like the Scooby-gang-is-made-to-suffer genre of dark sexfic as much as anyone. But I'm increasingly getting the impression that the Scoobies are being portrayed as cartoon villains in this story and that we're actually supposed to sympathize with their tormentor. That might make a certain amount of sense late in the series, when they've all done things they're not proud of. Season 3? Not so much. The murderess Faith having therapy sessions with the child-eating Mayor is especially incongruous.
Also, I don't know about the whole "They will suffer FOR EVER!" thing. I don't know about you, but I like these characters. That's why I watch the series. It's always fun to see your favorite hot TV babes being treated as naughty girls for a little while, but it's hard to see Willow in particular being consigned to hell for eternity. While the permanent aspect might be hot at the time, it leaves me at least with a bitter aftertaste. There are a few ways around this which wouldn't change the concept much. You could make this Faith's coma hallucination, for example (which would also explain why the Scoobies are so evil). You could have her eventually realize the Scoobs aren't so bad after all and draw a picture of herself in prison (restoring the late Season 4 status quo). And so on. I'm not suggesting that you pussy out and make the Scoobs like their degradation - I hate stories which do that - but throw us a bone here.
Also, diapers. That's just ... weird.
Finally, Xander. He knows the Scoobs better than anyone, and would make a perfect henchman for bringing them down. Look at what he did to Willow in the hyena episode. Faith, for example, is unlikely to know about Willow's public performance phobia - but Xander most certainly would. Best of all, Faith wouldn't have to use her pencils much if at all - she could simply get the various Scoobies to tell him themselves that humiliation turns them on, and that they want his help in devising scenarios to do so. And as long as he gets to take advantage of his friends, it would also fulfill her criterion of rewarding Xander.
Imagine a scene in which a bewildered and horrified Xander sees Willow performing, and tries to stop her. Faith forces her to tell him that being a slut turns her on, and, in fact, she'll let her best friend join the other guys in snaking her drain in exchange for a shiny nickel. I think that Faith would get a real kick out of that! Plus associating with Xander might convince her that the Scoobies weren't so bad after all. See above.