Not sure if this matters to you anymore but through various steps I came to the same conclusion you did.
Adding the user to Power Users will allow you to click and function within the Virtual Desktop. Through a bit of time and researching, the best I can gather is that Power Users is only there for legacy purposes, pre-Windows 7, and doesn't give more than a standard user (I can't find any real documentation stating it though, only some forum posts). Obviously it grants something as the desktop suddenly works with it. Here is one of those references though: https://www.quora.com/What-is-the-difference-between-a-power-user-and-administrator-in-Windows-7
Sum up is, I could not find a way short of Power Users to make it work. I reached the peak of time I was willing to devote. I *might* test a couple of minor things to see if I can find real differences in access between Users & Power Users though. Maybe there is some legacy style coding within the app that Power Users helps?
If you'd like, here are some of the things I tried in attempting to not join it to the Power Users group:
I used Sysinternals Accesschk.exe and couldn't find any additional folder/file permissions given to power users. I checked the Local Security Policy and found no additional rights given there either. I tried some application compatibility shims to make it work without Power Users but was unsuccessful. I monitored with Procmon to see if there were any access denieds or things easy to identify that might be interfering. I'm not sure which of the multiple Oculus exes operate the virtual desktop, I tried OculusClient.exe the Home...64-shipping one and one other.