ffmpeg: chroma key (greenscreen filter) a short video continuosly loopling into a live video

Use -stream_loop to loop and -re for real-time speed since you’re streaming (or else it will play super fast):

$ ffmpeg -stream_loop -1 -re -i input.mp4 -f v4l2 -pixel_format mjpeg \
    -framerate 30 -video_size 1280x720 -i /dev/video0 \
    -filter_complex \
        "[0:v]scale=1280x720[bg]; \
         [1:v]colorkey=0x00b140:0.5:0.0[ckout];  
         [bg][ckout]overlay,format=yuv420p" \
    -f v4l2 /dev/video1

Given that your inputs are YUV consider replacing colorkey (RGB) with chromakey (YUV).