Maybe you could simulate this by asking the reader to highlight on the page as they read (another commenter with dyslexia mentioned using this strategy)? Then hold the words immediately before and after the end of the selection constant. And maybe instead of randomly shuffling letters, swap out whole words with likely mistakes (given the number of dyslexic people in these comments, you could probably start with no substitutions and crowd-source a list of actual mistakes. To prevent the visual motion of swapping words from being distracting, you could make all non-selected words be very low contrast with the background. And if you wanted to be really fancy, maybe you could only make selection persist for a certain amount of time or words, so one can't simply select the entire document to fix it in place.