Transparent locked screen
Update: Transparent 2.4 works for external displays.

I have this question and tried the other answers.

The JohnnyNash and Transparent screen savers both work .. almost.  They show the main screen, but external displays go black.  The difference between these savers is that JohnnyNash is configured to update in seconds, and Transparent is configured to update in frames-per-second.

The Black Light screensaver boasts many features, but I did not see transparency on the list.

I found a screen saver called LookThrough, which boasts multi-monitor support.  It sort of works in that it displays on all of my displays, but the graphics are glitched up in weird ways.