Thanks as always Anup. I still live and learn. I admit that I tend to have SDIF locked in a compartment in my brain labelled " That’s what they do in Japan" ( where it is more commonly used in home audio settings) and overlook it 
. Incidentally does your USB /SDIF-2 converter provide the wordclock signal?
BTW, dCS Network Bridge manual indicates that settings to choose with Elgar+ in its compatibility chart should have DSD downsampling set to “On” which helped form my thinking.
Unfortunately it doesn’t help @melomane33 because, as you also mention, he doesn’t have the connectivity with that equipment combination. Had he then the answer to his question about how to use a NAS for native DSD would simply be that such files are handled ( served) in the same way as any other given suitable UPnP server software. Maybe he could find a used dCS Network Bridge but they are selling for top dollar currently.