We would have to use the Moodle API most likely. I could envision a JS customization like:
- on course home page, get the course ID out of the URL
- pass the course ID to a Moodle API to obtain its settings info
core_course_get_courses accepts a list of ids so it might work
- add visual indicator if hidden (e.g. a small "hidden" alert next to the course title, as on My Courses page)