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 outweigh that.
Well, one solution I can think of is this:
Replace Darth Sidious with Darth Maul in the federation related scenes. For the other scenes where Darth Sidious and Darth Maul converse, replace Sidious with another villain from the prequels, maybe even General Grievous (to address the issue of where the hell he was before episode 3 and to make it seem like there is a bigger conspiracy going on.) But replacing him with General Grievous could be a little problematic, as he is a bit taller than Sidious. (also one would need to rearrange the conversation in a convincing manner, also replace his voice in EP3 as well to match it, as he sounds like a cartoon villain in EP3 and thats not very fitting for a movie. He has the potential to be a great character, if someone can iron out the ridiculousness that is.)
Replacing/editing out Sidious as a hologram wouldn’t be too hard (as you would just need to replace the mettle in the middle and add a new hologram there) but replacing Sidious in those non-holo scenes would be difficult but I think it would worth it if someone actually menages to pull it off. (Though since General Grievous doesn’t have a visible mouth, it might be a lot easier to replace someone with him than anyone else.)