My vote, for what it's worth, is to put a human arm there. I always assumed a mistake was made in the original when it wasn't human. However, I suggest flipping the hand over so that the fingers are more visible and 'claw-like'. It would make the scene more gruesome and make up for the loss of blood, which I liked because it gave the scene an edge. Having it twitch would be a little over the top in terms of sheer nastiness. (WHOOPS! Jedi_jra said the exact same thing on the last page, with samples to boot!)
Again, awesome job Adywan, and thanks for the explanation about the lost Threepio reaction shot. I agree that the scene still works without it, as he does look a little forlorn at the news of Artoo's 'death', but it sounds like you have plans for this again...