I had a teacher that worked for the publisher and talked about how they'd have a series of responses for people who wrote in for the part of the book where the author says he wrote his own fanfiction scene and to write in if you wanted it.

Like maybe the first time you write in they'd respond that they couldn't provide it because they were fighting the Morgenstern estate over IP release to provide the material, etc.

So people never would get the pages, but could have gotten a number of different replies furthering the illusion.

I have a similar story from a different medium:

Frank Zappa has an album called Francesco Zappa. On the back of the sleeve, Frank describes finding out about a distant relative who composed and played music during the 18th century. After telling some friends about it, I got to thinking that Frank had invented another character (á la Ruben and the Jets), because that's the kind of thing he would do, and felt very foolish for repeating this information uncritically.

Years later I looked the album up on Wikipedia, and it turns out Francesco Zappa was a real musician in the 18th century (who was not actually directly related to Frank).

He got me twice with one album.