It seems that Multi-enrol is for 1.6. I am running 1.5. Besides the installation instructions sound sort of gibrish to me. I could check it out if it is part of the 1.6dev distribution in the downloads section of moodle.org.
I read the long thread about Multi-enrol and it indeed sounds like going into the right direction, although the focus is somewhat different than what I presented. It seems that Multi-enrol essentially allows us specifying multiple modes of enrolment for a given course. The discussion was about technicalogies it delivers whereas my focus was on user interface.
I am curious to see how what students see differs from 1.5.x. It is not clear to me how some of the distinction I mentioned are accomplished -- for example, the distinction between a course closed for enrolment, a course that requires manual enrolment by teachers, and a course that requires students to enter an access key.