Following a report by a Japanese blog, it can be confirmed by this publication that the PlayStation VR indeed works as a personal viewer for PC, Mac, Wii U, Xbox One, and other HDMI sources.
Tested by setting up the PlayStation VR as normal and attaching the headset to the external processor and plugging that into the mains, the HDMI input on the rear of the box accepts a video signal from any HDMI source, with the device acting exactly like it would when handling non-VR content from a standard PlayStation 4.
The one major drawback is that the processor box must be connected to the PlayStation 4 via USB, since it requires the Sony-specific handshake, and you can't just plug it straight into another system. In short, if one wants to use PlayStation VR with another HDMI system, they're still going to need a PlayStation 4 at hand - though the PlayStation Camera doesn't need to be attached.
The processor box pipes through the HDMI output as standard to your HDTV via the social screen output, while the view inside the headset blows up the HDMI feed to a cinema-size display right in front of your eyes. Head-tracking still works to a certain extent - you can adjust your view of the 'cinema screen' by looking around as normal. This appears to be handled by the processor box and the headset's sensor.
In terms of the quality of the presentation, the PlayStation VR's handling of the screen-door effect means that the image is soft, rather than pixelated or grainy. However, there's still a significantly reduced resolution overall.
The PlayStation VR can be used as a full VR solution for PC gaming as well. The interfaces used by the hardware are standard - USB 3.0 and a HDMI 1.4 signal that outputs 1920x1080 at either 90Hz or 120Hz. This does require a bit of reverse engineering, however, as the USB protocols need to be intercepted and reversed while the tracking solution would need to be rewritten from scratch to accommodate PlayStation VR's unique RGB lighting set-up and headset, DualShock 4 and PlayStation Move support. After all this the entire project would need to be integrated into an existing VR framework in order to make existing PC titles work with the hardware.
Overall, PlayStation VR's compatibility with other systems is a happy accident of the system utilizing an existing standard. HDMI transmitters can question the receiver to check that the right device is attached (which is why you can't attach another VR headset to the PlayStation VR's external processor), but there's no easy way for the processing box to lock out non-approved source. With that in mind, the PlayStation VR treats any standard HDMI input exactly as it would if it were coming from the PlayStation 4.
Despite all this, however, it's safe to say that the use-case scenarios are highly limited, and that it should go without saying that the device should only be purchased primarily for use with PlayStation 4 hardware.