I came across a reddit thread talking about using
Open Broadcaster Software to get a pseudo-mirroring effect. Not sure if it's been talked about in detail, but here's how I got it working.
I'm running my rift in Extended Mode, with it as the secondary monitor on the right side.
1. Download and install the free streaming tool OBS from
https://obsproject.com/.
2. Launch OBS.
3. Turn on the rift and launch the demo you want to use using the DirectToRift exe.
4. Alt-Tab back to the OBS window.
5. Right-click in the small "Sources" window near the bottom of the OBS window, and select "Add".
6. Enter anything you want as the name.
7. There are a lot of different options here, but from what I've tried, Game Capture seems to works best, so select that.
8. In the window that popups up, there should be an application dropdown at the top. Choose the demo from that dropdown and hit OK.
9. Hit the Preview Stream button in the bottom right.
10. Alt-Tab back to the demo window.
The steps above work about 90% of the time for me. Most unity demos work with this, but LFS and a couple others don't for me.