Right, that will give you the text at Loginwindow, but the OP (and I, google led me here) are seeking to also add the text to the lock screen. I'm trying to make it work through MCX via PM, but I've had a Casper customer confirm that he too can set the Loginwindow text this way, but text does not display while the authentication dialog on a machine that is locked due to sleep.
On a non-managed machine, sudo defaults write /Library/Preferences/com.apple.loginwindow.plist LoginwindowText "message" inserts "message" on both loginwindow AND the lockscreen. So far, no method of MCX does. I've submitted this as a bug to Apple.