Here's a hack to do it with xscreensaver and xdotool:
#!/bin/sh while true; do xdotool search --onlyvisible --name xscreensaver \ | xargs -r -n1 sh -c 'xdotool windowsize $1 1 1' - sleep 3 doneRun this and lock the screen. Every 3 seconds this script will try to shrink the xscreensaver window to 1x1 pixels and reveal your desktop below it. Useful if you need it.