It's already possible to register on a closed site, though it hasn't been made obvious to help prevent spam.
If you click the Login button, you'll get the log-in form, which also has a join link. You can go through the full joining process even on a closed site. You just can't log in as a new member afterwards yet.
If you click the Login button, you'll get the log-in form, which also has a join link. You can go through the full joining process even on a closed site. You just can't log in as a new member afterwards yet.