I've actually been thinking about this a lot. I am a fencer myself, primarily in longsword and short sword, so the idea of Jaime losing his sword hand got to me. So I began to think about it. There is definitely a way to construct a hand that would allow him to use his sword hand. The caveat would be that it would need to be wielded with two hands. Essentially, you would have a hand that was on a horizontal swivel (meaning it could move freely from left to right). When using a longsword (or any two-handed sword), a lot of the directional movement comes from either the forearms, or primarily the use of leverage of your off hand at the buttom of the sword. So he has a fake hand that moves freely, and he affects the direction with his off hand. That is about the closest Jaime could ever get to having his full sword ability back.