Using the latest 0.57 version of Configurator, you could easily make a layer, that will appear only when system's screen-saver is running.

(Since 0.59 version of Configurator, ScreenSaverEngine is added automatically to this list)

1) First of all, go to Finder and press Cmd+Shift+G and navigate to this directory:
/System/Library/Frameworks/ScreenSaver.framework/Resources

2) Open Configurator's preferences (Cmd+Comma).

3) Drag-n-drop ScreenSaverEngine file from that folder to the "Custom applications" table in the Configurator.

4) Create any layer at the top of the stack and assign ScreenSaverEngine application condition to it

Voila!