That is why I would suggest rotoscoping the eyebrows alone and re-colouring the eyes using After Effects. I don't know if you would be able to see a composite is even there if it was just the eyebrows. But if you are using the GKAR for the whole shot, I don't know if the cross-cutting between Luke and Vader switching between two clearly different sources would work. Of course, I guess you would have to see it to judge it, I know GKAR isn't as bad as the GOUT, but to me in theory I would expect the cross-cutting to be a little jarring. It is, of course, much easier than doing AE work, so I guess it depends on how much time you are willing to invest.