brash_stryker said:

I wonder if Mike Stoklasa would be interested in just saying "Luke" if the Ian McDarmid clip isn't sufficient. He does a mean Emperor....

You may have been joking, but actually that's not a bad idea at all.

Just as Ady was on a search for someone who can digitally replicate an explosion for the generator, he could go on a search for someone who can do a really good emperor voice.. get a recording of them saying "Luke" for this scene, then seamlessly fit it in. This may be the best option, instead of using a "Luke" said by Ian with a different pitch/tone.

And of course keep this same person handy when it comes time for doing the prequels/ROTJ, just in case there's changing/adding/improving of dialogue ;)