[Plugin: WPMU Blog Name Restrictions Override] Blog name restriction in WP3

Before WordPress 3 I used the WPMU Blog Name Restrictions Override plugin to set a limit on blog names to no less than 5 characters. It doesn’t work for me now. Users can ignore my rule and register 4-character names because I can’t restrict it anymore.

Is there any way to do this? I’ve tried messing around with wp-login.php but no luck.