[enjoylife]

Sure, not sure what gstreamer plugin/sink would create a loopback device, but this plays as well.

   gphoto2 --stdout --capture-movie | ffmpeg -i - -vcodec rawvideo -pix_fmt yuv420p -threads 0  -f matroska - | gst-launch-1.0 fdsrc fd=0 ! decodebin !  videoconvert ! videoscale ! autovideosink

[ceocoder]

per this - https://apple.stackexchange.com/a/356362

it should be,

  osxvideosink

I wonder if this works,

  gphoto2 --stdout --capture-movie | ffmpeg -i - -vcodec rawvideo -pix_fmt yuv420p -threads 0  -f matroska - | gst-launch-1.0 fdsrc fd=0 ! decodebin !  videoconvert ! videoscale ! osxvideosink