There are two separate issues here, 1) DSD digital interfaces and 2) Playing DSD on MC
1) AFAIK after further exploration since my initial post there was/is no standard digital interface for DSD.
Sony released an interface known as "SDIF-3" (not to be confused with AES3 or SPDIF).
Some early DSD equipment apparently used a data format known as ‘DSD-raw’ or "SDIF-2" which was apparently simply a stream of DSD samples in non-return-to-zero (NRZ) form. I do not know if either of these exist in the real world today.
I am aware of DACS said to be compatible with DSD transfers over USB, Firewire, AES/EBU, S/PDIF and HDMI I think. I am not sure if these use a DoP implementation or how many can do raw native DSD bitstreaming. There are also propriety implementations talked about in SACD players.
2) DSD on JRiver MC. I know, as Matt points out, MC can accommodate DSD over DoP (this is not the same as converting DSD to PCM, which MC is also capable). I have used DoP over USB with an Auralic Vega DAC I auditioned. This required an ASIO driver from Auralic. The native/raw DSD bitstreaming, again as Matt points out, is over an appropriate ASIO driver and a different setting within MC. I gather the driver would need to be supplied by the DAC manufacturer that support this implementation and whatever particular digital interface connection method. Some MC related DSD links
http://wiki.jriver.com/index.php/DSD_Format http://yabb.jriver.com/interact/index.php?topic=65161.0 http://yabb.jriver.com/interact/index.php?action=printpage;topic=66719.0 http://yabb.jriver.com/interact/index.php?topic=80983.0;topicseen