I have an idea and am not sure I'd be able to do this.
For enrollment's, I know that you can set a course to expire after so many days could that part of the program be copied into the enrollment plug-ins where you can set an expiration from when the user enrolled on your site, not the class? When that happens they can renew their enrollment.
That would solve the problem with moodle users wanting to have an open enrollment type system where the users enrollment expires, not the classes.
Just a thought.
Michael