It's correct that the Oppo HDMI inputs do not accept DSD. It's also true that some HDMI outputs on Oppo's can transport DSD (on the BDP-103 and BDP-105 only HDMI2 out can, on the BDP-103D and BDP-105D, both HDMI outputs can).
However, most, if not all, PC video cards are not able to output DSD via HDMI. For example, my Denon AVP-A1HDCI pre-pro will accept DSD via its HDMI inputs, but I can't connect my PC to one of the HDMI inputs and send DSD to my pre-pro as the limitation is on the PC side.
On second thought and although I disagreed in another post, I think you may be right. As I think through my old experiences, I never actually did use a PC for bitstreamed DSD output via HDMI, just USB. The HDMI protocol itself does handle bitstreamed DSD, but apparently not from a Windows PC because the HDMI audio output via GPU or integrated graphics will not handle DSD. Upon reflection, my experience with DSD over HDMI had been entirely sourced from an Oppo player playing an SACD disc, not from a PC. Sorry for the confusion, but I do not want to misdirect any other readers of this thread.
So, I believe bitstreamed DSD from a PC is only possible via USB, as you indicated. (There is also DoP, which I am guessing could theoretically be done over HDMI, although I am not certain. That puts DSD in a PCM wrapper, but the receiving DAC has to support that and most do not support DoP, even if they do support bitstreamed DSD. DoP is primarily used with digital coax or USB.)
For the past 3+ years, my own listening to SACD rips has primarily been from DSF files converted on the fly to 88k or 176k PCM by JR and using Dirac Live software. Initially, that was via HDMI into a prepro. But, for 2+ years, it was via USB into an Exasound E28 DAC. USB and the DAC also allow me to listen to bitstreamed DSD, which I do only occasionally. Most all of my listening is in 5.1, occasionally in 7.1 from BD. Incidentally, I am extremely happy with my sound.