Use gstreamer to create a dummy V4L2 device combining a still image and live video by turning a greenscreen background into an alpha channel.
You'll need to install the v4l2loopback kernel modules.
sudo apt install v4l2loopback-dkms
Then create the loopback device. In my case it ends up as /dev/video2 because I have two
other devices.
sudo modprobe v4l2loopback