A simple idea which might get us a long way:
Add two new fields to a course:
- course value (eg credit points)
- course pass grade (value between 0 and 100%)
When the total course grade percentage for a student exceeds the pass grade percentage, then the student is assumed to have passed that unit.
This status can be copied to a new table on a regular basis, so that even after unenrolling the record remains there.
On My Moodle, and also on a new admin report screen, all these values can be displayed to give a course record for that student, with a total of credits.
Simple but should meet the needs of many people.
(Full specs from 2009: http://docs.moodle.org/en/Development:Course_completion)