I have the same desire. I would like a way to mark myself as low priority to get into a game, so I never accidentally stop someone else from getting a game (when I'm just trying to make teams even, so no-one is stopped from getting a game by the minevenplayers setting).
Currently, to do this, I need to leave and re-join the player list every time someone new joins the lobby, and even this fails sometimes, due the the new rules where people who played in the previous game are de-prioritised.