The claim that you can find any text in DNA, the Communist Manifesto or the entire works of Shakespeare is simply not true.
I believe you are right there. The downside is that I also believe that you won't find a large part of the bible either.
And reading a file backwards is not quite as simple as simply calling reverse, because what you do in your example reads the whole file into memory and that will not work with very large files.
And as you still seem to be in the brainstorming phase:
You probably know that only a small part of the dna is actually encoding proteins, so you may consider restricting your search to these areas (or maybe a restrict your search to the non-coding parts).
Lots of parameters you can tweak, so you'll bound to find something...