I agree completely. However, even masking off the face area and dropping the lighting to create a blacked out hood and keep the effect looking neat would take an awful lot of work. Whilst I would like to think it could be done another way I’m not even sure of another method which could be applied. Even with an actor like Christopher Lee who has been in so many movies, I’ve still been struggling to find any shots which could be convincingly added to the jedi scenes to make him look like part of the council. I think then trying to find more shots of him to replace Sidious would be impossible (unfortunately). Though if Sidious’s voice were changed then rather than just running the existing voice through a filter, whole new lines could be made without the worry of matching lip movements. Again not a perfect solution because it would look a little cheap, but perhaps one where the benefits could outweight that.