First, ASRock states that the wasted memory feature works with 32bit OS's. I've never used a 32 bit OS with XFast RAM, so I'm not 100% sure how it works.
The main question is does WHS 2011 see the 16GB of memory? Does it recognize it? I assume the UEFI/BIOS sees it. If WHS only reports 8GB to XFast RAM, then it cannot use what it is not aware of. 32 bit Windows will see the memory beyond 4GB, but cannot use it.
Unless someone knows how this is otherwise done, I don't think you can.