Sorry, the question had me weirded out...
If the background of the shots are identical, ie. an immobile camera, then you can probably composite multiple shots of the actor. With the legs staying still, though, chances are you'd shoot that as a separate element, perhaps even use a still. But Final Cut really wasn't designed to do anything as advanced as this -- if you're trying to approximate a 'draft' version of the effect, all well and good, but if you're trying to make it look as slick as in The Matrix, I don't think FCP can do it. It just doesn't have enough options on the effects front.