can you try piping that to gstreamer?
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 ! autovideosinkit should be,
osxvideosink
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