The ability to lock an individual league tier would be very useful in order to allow access to certain level, whilst restricting access to others.
As an example to make things a little clearer. I have a strong league with a very active Elite tier and have an active waiting list of players wanting to join. In order to process my waiting list, currently I have to password protect the entire league until I have the waiting players join. The downside of this being that my lesser active Rookie tiers build more inactive players that I cannot refill until the lock is dropped from the league. Being able to restrict access to my Elite tier, would allow the free movement within the Rookie and Pro tiers (which have a natural high drop rate) and allow the waiting players entry when they are free to join (such as the completion of a season they are currently running).