This is one possibility I proposed to RMS. Essentially, I proposed that all Invariant Sections had to be placed in debian/copyright, and that any duplicates of his Invariant text in the "actual" documentation would be modifiable.
That is unacceptable because it would allow modified versions of the GNU Manifesto. The GFDL, like the previous Emacs Manual license, was designed to require every copy of the Emacs Manual to include an unmodified copy of the GNU Manifesto. I hope that Debian, if it adopts a version of your guidelines, will modify them so as to accept the Emacs Manual.