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.
Had to go to ChatGPT for help. It appears that you need to know how many tiles to stitch. I was hoping to have that dynamically determined. Not sure if loopback will help.
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