Waitlist for over-capacity sign-ups

When I set a sign-up capacity limit on a course or event, I want to be able to capture additional interested prospects to a wait list. I could use the waitlist to fill the course or event if people drop out, or notify the waitlist of a new cohort or event etc

The workaround now would be to have everyone sign up on the wait list, and then manually approve, say, the first 20 people who have signed up.

I'm looking for an automatic way of approving those first 20, but then putting subsequent registrations onto the wait list with a notification.