The way this watermarks are usually done is to put like white text on white background so for a visually impaired person the text2speak would read it just fine. I think depending on the word processor you probably can mark text to use with or without accessibility tools, but even in this case I don't know how a student copy-paste from one place to the other, if he just retype what he is listen then it would not affect. The whole thing works on the assumption on the student selecting all the text without paying much attention, maybe with a swoop of the mouse or Ctrl-a the text, because the selection highlight will show an invisible text being select. Or... If you can upload the whole PDF/doc file them it is different. I am not sure how chatGPT accepts inputs.