The sections added to the GNU FDL document have nothing to do with "fair use"; they are original work.
You misunderstand, I wsa trying to give an example which the "fair use" doctrine could be used to exploit the GNU FDL. I could write a document on my computer and then incorporate parts of that document as "fair use" into the GNU FDL articles, no-one else would then be able to modify those sections as they wouldn't be licenced under the GNU FDL.
Let me see if I'm misunderstanding this correctly! Are you suggesting that you could take an extract from your own previously unpublished original work and claim that the extract was subject to "fair use" rules? IMHO it seems that fair use doesn't apply because you already have the permission of the original author. Who could you (as author and copyright owner) possibly sue for copyright violation? The simple act of putting the material on Wikipedia is what would trigger the application of GNU FDL. Your reasoning would make it impossible for anybody to edit anything at all on Wikipedia, and that seems contrary to the basic intent of the project.
Rclecticology