I agree with Chris. A button/checkbox/flag of some sort would be very useful.
Currently there are only three states a student can be in with regards to enrollment: enrolled, unenrolled, and suspended. With the external database, we set it to unenroll students because we do not want them showing in the Moodle course if they drop the course. However, we also want to be able to block course access for students who have stops, holds, or suspensions on their account, and we can't do that automatically. Being able to hide/show suspended students would let us be able to set all enrollment changes to suspend instead of unenroll, so faculty can still see submitted work from the students with the various stops.