Is there a way to start the screensaver (and lock the screen) from a bash script?
Have you tried this?
/System/Library/Frameworks/ScreenSaver.framework/Resources/ScreenSaverEngine.app/Contents/MacOS/ScreenSaverEngine
Update for macOS High Sierra:
/System/Library/CoreServices/ScreenSaverEngine.app/Contents/MacOS/ScreenSaverEngine