I've tried a variety of different ways to change the message for the "message" screen saver on macOS 10.10.3. When I change it through the GUI, at least the following plist file is updated:
When I try to update this file as root using:
defaults write /Users/myusername/Library/Preferences/ByHost/com.apple.ScreenSaver.Basic.UUID.plist MESSAGE "test"
defaults write ~/Library/Preferences/ByHost/com.apple.ScreenSaver.Basic.UUID.plist
after closing the prefs window, when I subsequently re-open the sys prefs and view the screen saver settings, they still show the old settings.
The following command to restart the prefs engine doesn't work either:
If I modify the above files using defaults without opening the system prefs, then restart, strangely, the screensaver reverts to the default computer name after the restart.
Ultimately I'm trying to enforce a custom screensaver with floating text on all my macOS machines.