I have not tried it with Zoom but Chrome refused to recognize any loopback video devices unless they were capture-only. So the following worked:

    # modprobe -r v4l2loopback
    # modprobe v4l2loopback video_nr=7 exclusive_caps=1 card_label='Screenshare'

where exclusive_caps=1 is the work-around for Chrome (both video_nr for /dev/video7 and card_label should be able to be set to ~arbitrary values). You need to first start writing stream to the loopback device and then it would switch itself into a capture-only device and Chrome will recognize it.