I expected to find Lockscreen option in CCSM after upgrading to Ubuntu 14.04, in Unity Plugin, general section, in order to switch between unity and lightdm. Unfortunately, It's not in the tab.
I have looked around, even in the source of Unity, and it does not seem like there is a way to do this.
Obviously, the Ubuntu developers don't share the opinions in http://www.jwz.org/xscreensaver/toolkits.html, but they should respect that others might do, and not prevent users from choosing their own screensaver. Several bugs has been found already in the new lockscreen.