I stumbled across wallset on github https://github.com/terroo/wallset
This uses ffmpeg to create wallpaper images for 10 seconds from a video. And then lets you play them back smoothly as video wallpaper. I have it working, on fatdog ... but I only get a glimpse of it when logging out! It seems to be 'behind' my wallpaper. Maybe someone knows what i need to do to see that 'video plane'.