You CAN download it on Ativ S. Go in the topic "Latest Ativ S firmware updates" and in last few pages there is guide to edit registry and pretend it is Lumia and download.
I've done that sucessfully and don't recommend it, had multiple problems, black display, freezing,etc.in just a two hours of use. Not worth the hassle for now.
This specifically worries me. If you're tricking the software to think that it's working on Nokia hardware, problems should definitely be expected.