You don't need a loopback decoder for that. The periodic extraction will depend on a filter, and you can just clone and send the output of that filter to the tiling filter.
CGPT said: ffmpeg -i input.mp4 -vf "fps=1,tile=3x1" -frames:v 1 output_stitched.png
Gemini: ffmpeg -i input_video.mp4 -vf "fps=1,scale=220:-1" -c:v png output.png