I've just submitted a readability analysis filter to Moodle.org plugins repository for approval.
It analyses text using one of 6 popular readability analysis algorithms and prints out the results at the bottom of the text, with a link to the respective Wikipedia.org page explaining what it is and what it does.
It's useful for monitoring the complexity of language people use, particularly teachers who sometimes have a tendency to write overly complex and difficult to read comments. It might also encourage learners to "join up" their ideas and relate them to each other in longer sentences, e.g. using conjunctions and clauses.
When/if it's approved, it'll be available at: https://moodle.org/plugins/view.php?plugin=filter_readability
If you can't wait and want to see it now, it's also on my Github.com repo: https://github.com/matbury/filter_readability
I hope you find it useful