As much as I respect the originals, I really don't think it'd be a good idea to use Shaw.
People always argue that Shaw IS Anakin Skywalker, but... frankly, the Prequels WERE made, and Hayden WAS cast as Anakin. Hayden is Anakin Skywalker, and for the sake of the Saga, it should be Anakin Skywalker who appears at the end.
Shaw played an important part as the man behind the mask - but his voice and appearance really shouldn't be the basis for the formation of the Anakin character - considering we only can go on what we have in the movies, which is only about a minute or so of an old, broken man's labored last words.
I really don't see it as disrespect to "rob" Shaw of the last few seconds he appears as a ghost. And for the sake of the full-circle of the Saga, it probably should be Hayden.
BUT! It should NOT be the Hayden we see in the DVD and Blu-Ray. They should have made him look as old as he's supposed to be (What would it be? Like 50?).
So I'd suggest maybe taking Hayden's head from another of his movies where his hairstyle isn't so rouge-like, and maybe grey him up a little bit digitally. If you really want to go the extra mile, add in some wrinkles and you're set.
And concerning a scene about the stolen plans...
I really like the idea of showing it, but I'm just a little afraid of how it affects the style in relation to other Star Wars movies. This is the kind of scene you'd see in a cold opening, which is a lot more "modern" than the overall retro style of the Star Wars movies.
Is there a way to incorporate this idea more into the flow of the script?