I'm still relatively new here, so correct me if I'm wrong, but there's nothing stopping you from creating your own server. There might be lag issues if you have a lot of people in the server, but it wouldn't be too bad. You could lock it and PM the password to the pending people. The only bad side is that the server would disappear when the host closes the sim. A way to get around this would be if someone got in the lobby and saw there was no "pending people" server, he would just create one and when he leaves, someone else creates the server using the same name/password.
However, like Richard explained, there may be wreckers among the pending people, but you could just change the password and not tell that person.