You can always join a raid lockout as long as there aren't any bosses up you've already killed (so you can't get loot off one boss more than once per week). Unless you kill a boss on heroic - in that case you can only join another raid if they do the remaining bosses on normal (it won't let the RL convert to heroic if someone in the raid has a different heroic instance ID). Which is a huge pain in the ass IMO :P