Hell, the way Pratchett eats through pop culture for material, I’d figure we’ll see a Discworld book featuring Larry Kotter, Teen Wizard, Heart-Throb, And Bestselling Novelist, any day now…
I remember that thread (although I can’t find it either), and I think that was a “wouldn’t it be cool if…” idea from a poster, not something from an actual Discworld book. I don’t recall seeing it in any of the Discworld books, and I’ve read them all several times.
Besides, what self respecting wizard would be caught dead on a broomstick?
Ridcully flies one (hauling a naked Sam Vimes around, no less) in Night Watch. And if you made a list of “self respecting wizards” I think he’d be near the top of the list. 
I seem to recall some buzzing around near the end of Moving Pictures, though maybe it was only the Librarian.
Oh ok, Miller, I thought it had actually happend, Ive not read all of the Discworld novels so I didnt know…
Thanks!
No, widdershins is not only in the Terry Pratchett book. You may not know this book, but it is called “Carbonel, The king of the cats” and the author, I forget her name. Rosemary Brown, the impromptu witch flies widdershins around the cauldron while it is boiling hot.