Idea is to capture parts of the desktop and play its contant back in a windows. Then this window can be shared (e.g. in Google Meet).
Ubuntu needs and VLC with an additional packange:
sudo apt install vlc-plugin-access-extra
Then run VLC to capture and playback the screen region in a window that is to be shared:
cvlc --no-video-deco --no-embedded-video --screen-fps=20
--screen-top=0 --screen-left=0 --screen-width=1920 --screen-height=1080