Why not have a "clear required" checkbox? As others have stated quite effectively, it wouldn't signal any huge shift in player behavior. As soon as the bonus myth notification comes up, you know someone was lying and probably hoping to be carried if they joined a PF obviously labeled as "farm only/must have cleared/etc.". And yes, some players actually may know the fight perfectly and not have a clear thanks to grouping with players who can't perform the mechanics, but that's where you communicate your desire to join the group and let the leader decide to open it or not.

When it comes to end-game content, elitism isn't the only reason for not wanting to teach people from square one; sometimes it's just a matter of how much time you have to play. If you want a learning or practice party where you dedicate hours to getting everyone up to speed on mechanics, that's fine, but not everyone has that kind of time every day. I'll help with learning parties when I can, but sometimes I just need to get my own things done (especially right before reset) :P