I think this strongly depends, how the MC-ed players targeting mechanixm works.
if they behave like having an own threatlist (tauntable?), then I agree that some basic ControlledPlayerAI needs to be implemented into core;
however it also is possible, that they behave like an extention of the Boss' AI, and in this case I think we might get good results with an (generic?) AI-box which bosses can use(*), and which handles the MCed players' actions
This might be usable like (PlayerControlledList Player*) DoHandleMCPlayer(Player*, uiDiff, m_creature); - which means, we would not have to worry for complicated script assigning, etc