I believe this is going to require programming unless something has just been added to core. (I'm using 2.2.)
I've been editing some enrolment plugins, so I know how to do it on an enrol-method instance level, but it sounds like you need it on a course-level.
At the enrolment level, look at the messsages in https://moodle.org/mod/forum/discuss.php?d=189334 starting on March 27, 2013.
I haven't made any course customizations yet, but I think what you would need is to modify the course edit functionality to include a new field (like enrolment instructions) along the same lines as the course description. Then in enrol/index.php, before displaying all the forms, get the message for the course and display it.
Michael