I purchased the headset from Walmart. I have a large head and I am super duper near-sighted (myopic) so one thing I noticed almost immediately is that this headset can't be worn with glasses on (unless you are good with a Dremel and can widen the eyepiece area carefully and re-pad it)

I also went with a different screen that has HDMI in and audio out on the board. I have ADD and would like to see if this could help me focus. Rather than being able to mount the pi on the back of the screen, my plan is to just mount the screen to the goggles and then run the cables for power and video signal over the central headstrap into a pack I can wear on my back or chest. I know it gets a bit messier this way, and I'm still  not sure this is what I will ultimately do, but it'll give me the option to plug it into a laptop or other video source. What you did with the pi though is great, it adds almost no extra weight. The tradeoff is that the USB ports could be hard to get to and/or you don't want cables hanging as much as you can avoid it.

I've been thinking about HMD for PC, but all of the cheap smartphone headsets I've seen use 2 lenses internally to accommodate split-screen "Google Cardboard" type applications.  Is this headset different off-the-shelf, or did you have to modify it somehow?

Me too! From the video, 1:30 gives a glimpse inside and shows that it's one big lens, rather than two separate ones. So no 3D effect is possible, but it's much simpler to configure.

That's partially why I did it. Getting the 3D effect on a Pi was overly complicated but the large lens allows me to get a great view and still get work done. I watched a full movie on Netflix with the rig and I have to admit i was like being up close at a movie theater.

Lack of 3D effect is hardly a bad thing, when it comes to just being a "regular PC" er cyberdeck :)  As Jordan points out, it really simplifies the software layer.

Even knowing that such a headset exists, I can't seem to find one on Amazon :(  Maybe will be easier to find than some high res 2" LCDs and an hdmi splitter though :)

This worked the way I needed it to out of the box. It's missing the monitor that comes with it so I just put my screen on top of it. The sizing wasn't exact so I used super glue tape to make sure it stayed on. That worked wonders. The only glitch was the original goggles had latches on it but those were easy to cut off.

Some specs on the Walmart-distributed headset, in case they help you.

Part numbers: 22630, 13056(A, B) in the castings

Brands: Toy State, Nikko. Made in Shenzhen but the company is based in Netherlands.

Keywords from the packaging:

The opening for the display is 60x100mm, but could be dremelled out to about 75x120 without any damage to the case. There would be some distortion from the edges of the lens, but it would work.

Inside is just a simple fresnel lens, rectangular. You can purchase one (sold as a "page magnifier") and build this whole thing yourself.