What you could do, is make it so the windows are all independent, and then move all the UI to one side, but you would still have the video on that side as well. It will free up the left side (or whatever side you want) for all the viewing you want. Just do most of your fighting from the left.
But yes, I agree that we need a /viewport or horizontal slider like people before me mentioned. That would make the game so much better. The Letterbox just doens't work well for that kind of stuff.