Seems to be a misunderstanding of Filevault vs login screen. When you reboot or start-up a Filevault enabled Mac, the first screen encountered is actually the Filevault unlock screen. If the username and password entered there match that of a local filevault enabled account, then the actual log-in screen is bypassed, taking the user directly into their account/desktop. If you have ever seen where the Filevault and local password is out of sync, then you will see where the FV password is used on the first screen, then the user will be presented with an actual account log-in screen (where you would see your log-in screen message) to enter a different password since Filevault and the local account are out of sync, probably from changing the password incorrectly (happens more with AD bound Macs). Here you would see the log-in screen message you are trying to display. If you log-out of the Mac, not restart, then you will be presented with an account log-in screen to get back into the account and the Filevault unlock screen will not be used since Filevault is already unlocked, unless the Mac is configured to lock Filevault at log-out. So to make a long stort short, you cannot configure a message to be displayed at the Filevault unlock screen, only the account log-in screen. So unless Apple changes the ability to configure a Filevault unlock screen message, which would actually have to be stored in Firmware I believe, there is no way that a log-in message will work with Filevault enabled Macs.
