FFMpeg: scale and then overlay keeping overlay aspect ratio
Ended up solving it through scale2ref:
ffmpeg -y -i video.mov -i watermark.png -filter_complex &quot;[0:v]scale=-1:1280:flags=lanczos[bg];[1:v][bg]scale2ref=(250/180)*$ih/8/sar:$ih/8[wm][base];[base][wm]overlay=(main_w-overlay_w)/2:(main_h-overlay_h)-overlay_h&quot; -crf 0 -preset ultrafast -c:v mpeg4 -qscale:v 10 -c:a: copy out.mp4`

Where 250 is the width of the overlay in pixels, and 180 its height.
This outputs a video of N:1280, with an overlay of 250x180 at the bottom-center of it.