Ha, thanks for the info, but I'd suggest you give it some more thought. There is NO reason why a cast wheel couldn't have the disc mounted. It has a hub just the same as a wire wheel, although it's all one piece of metal with spokes and rim.
However, that's not the real question. It is obviously (to me anyway) beneficial to have the disc staying with the Final Drive and NOT being attached to the wheel. This makes wheel removal and re-fitting a lot easier. So why did BMW NOT do that with wire wheels? Even if as you suggest they are ideally suited to have the disc mounted on them, why do that when it's a far better solution to have the disc mounted on the final drive?
In fact that rather begs the question of whether a wire wheel (without disc) can be fitted to a Final Drive that has the disc mounted on it? Is it in fact a choice as to where the disc is mounted? On the Final Drive or on the wheel?