Whoa, that's awesome! I tried to get the same thing working with ffmpeg, a la:

  ffmpeg -device /dev/dri/card1 -f kmsgrab -i -
    -vf 'crop=4096:2160:0:0,hwmap=derive_device=vaapi,scale_vaapi=w=4096:h=2160:format=nv12'
    -c:v h264_vaapi -qp 24 output.mp4

... and that process was, um, exciting, but not in a good way if you care about having an MP4 at the end with minimal fuss.