Just to complete the loop I've just had confirmation direct from Netapp that R-D2 is supported on mixed storage system.  The "all-flash" requirement _only_ refers to the initial shelf/root volume.  As long as that requirement is satisfied you can add whatever type of shelves you like.  So FAS with a shelf of R-D2 SSDs can totally have SAS or SATA shelves added to it later.  Now why they can't spell this out in a TR or communicate in a way that doesn't feel like pulling teeth is a mystery to me...