Use Phone as "Window" for SteamVR?

Hey!
Does anyone have an ideas as to how I would be able to set up a mobile device to act as a wireless viewer for a SteamVR game?

The best way I can explain is is kinda like if you play a 360 degree video on YouTube without a cardboard and it uses the gyroscope to look around (I think Google calls this a "Magic Window").
I want to somehow have a second player "join" the VR world using their mobile device like that.

Does that make any sense? If So, any ideas?
Thanks so much!

 

Stick a Vive tracker on the back of the phone, make a standalone app on the phone that sees through a camera in the game scene?

 

Right, so I'm trying to avoid using any extra hardware...

I'm gonna try using the AR Foundation to track the phone's position in the world (resetting the tracking session in the real world position/rotation of the game scene camera--if that makes sense).

But I have no idea how to send over the transform data from the phone app to the PC, let alone how to stream the camera image... Maybe a render texture? Still same problem, how would I stream it without a huge hit to the CPU?
Any ideas?

Thanks so much for the reply!