For a professional, skilled forger with resources, I don’t think there would be any particular difficulties, just attention to detail and nuance. A national intelligence service could do it.
The problem is, professional faking a document like this is a waste of time, because it is essentially a treasure map. If you follow the instructions to the end, (i.e. to book 44B, page 5733) and there’s no treasure there, the skill of the forgery is all for naught. And for all the resources of the Mossad, the CIA, or others, I see no way that they could go back in time and see to it that a specific page in a 47 year old bound book could be bound in invisibly. And what of the page that is supposed to be there? Where did it go? The index would no longer match, so it would have to be corrected too, and that is another monumental task that would have to be carried out right beneath the noses of the archivists.
Very difficult, bordering on the impossible. The system is designed the way it is to prevent just such a thing happening.
For an amateur, I don’t think they would stand the remotest chance of carrying it off, even the document itself.
An amateur would wind up with something as obviously fake as the Bush National Guard letters. This document is NOT obviously fake.
Bond, James Bond.
Que the Goldfinger music.