Wouldn't it be possible to just show the computer screen on the VR headset (without stereoscopic vision, just using the headset as a monitor), and that way bypass the fact that most VR systems don't support D3D9?
Regarding the camera movement, the API only allows for rotation in polar and azimuth axis, but I would think that's not a huge issue. One does not normally rotate the head side to side. If you do, it wouldn't respond to that movement, but I would think it could still feel realistic.
Holy crap, I just found this video on youtube which was published literally last Monday. He is using the TrackIR plugin for tracking, but using some software to get the position/rotation data from the Occulus itself instead of a TrackIR device. Then he uses SteamVR to show the image on the headset; and it even works in stereo??
So it's already working?? This seems way too good to be true.