I'm confused why you focus on plagiarism detection. That being said, your scenario is very briefly mentioned in the conclusion and requires augmenting the approach (entropy coding) with error correction.

The result would be that as long as your modifications (reordering clauses, etc.) reasonably closely follow a known distribution with limited entropy (which I think it clearly does, although specifying this distribution and dealing with the induced noisy channel might be very hard), there will be a way to do covert communication despite it, though probably only a very small amount of information can be transmitted reliably. For plagiarism detection, you only need a number of bits that scales like -log[your desired false positive rate] so it would seem theoretically possible. Theoretically it also doesn't matter if you use text or images, though in practice increasing the amount of transmitted data should make the task a lot easier. However, I'm not sure if something like this can be practically implemented using existing methods.