Add single pixel to .gif file using ffmpeg
Solution 1:
Adapting How do I convert a video to GIF using ffmpeg, with reasonable quality? by adding the drawbox filter:
ffmpeg -i input.gif -vf "drawbox=x=0:y=ih-h:w=1:h=1:color=red:t=fill:replace=1,split[s0][s1];[s0]palettegen[p];[s1][p]paletteuse" output.gif
If the pixel does not show up increase the h value or you may have to adjust the options for palettegen/paletteuse.