The current workaround for this would be to use an app like IPWebcam and route the network traffic over USB with adb, then ffmpeg it into v4l2loopback on your computer.

I have been using an old smartphone as webcam for quite some time now, and I got to say it works quite well. The only problem is the power supply, because video recording draws a lot of power it drains the battery even when it's plugged in.

Of course, an out of the box solution with UVC would be much nicer!