If we're talking about what's optimized for eyes and the movement of them, you wouldn't even want a keyboard layout, you'd want something more like a dart board or oval shaped dartboard where the spaces are letters and the most common letters are towards the center. QWERTY is fine for someone familiar with it, like an experienced typist whose already mapped those keys mentally, but I expect someone with a lifelong disability would have no true familiarity with QWERTY (since they can't type) and would have to learn any layout from scratch anyway, so you might as well start from scratch.