Reducing refresh for video viewing is not something I would recommend.
You are not so much displaying a film in the headset, but placing yourself in a virtual theatre.
Even if that virtual screen is just a screen in blank space.
If that immediate game world is behaving less like any other VR experience, people will get sick.
Even when sitting still, staring at a virtual screen.
What I find best to do when testing this in the Vive and CV1, was to use a virtual desktop software, like BigScreen in combination with SVP, SmoothVideoProject to increase the frame rate of the movie and reducing the fps scaling errors.
I'm sure somewhere someone will make a projection to work at source HZ, but the game world and the HMD itself need to remain at it's designated high fps.
This is not an issue Pimax or any HMD manufacturer should bother themselves with, this is for the developer of the video player.