I'm actually amazed that this isn't already implemented, if it is than i'm sorry for posting about it. I think players should be able to set their status to invisible in-game, so that they done appear online, but they can still get PP. This would be especially useful for global moderators, for obvious reasons.
Now, i also think that while appearing invisible, you should be able to initiate pm conversations, but not be able to have them initiated with you. This would make it so that you could still talk to a friend if you wanted, but generally if you wanted to be left alone for a while you could still do that.
The forums already has a similar feature, so i don't think there should be any particular reason why this shouldnt be added
Now, i also think that while appearing invisible, you should be able to initiate pm conversations, but not be able to have them initiated with you. This would make it so that you could still talk to a friend if you wanted, but generally if you wanted to be left alone for a while you could still do that.
The forums already has a similar feature, so i don't think there should be any particular reason why this shouldnt be added