@davidedpg10
TL;DR: Messages on the user desktop could be done, login screen is not possible. However, FOG 1.3.X is no longer adding features as FOG 2.0 is under way
Detailed explanation:
In the days of Windows XP what you want would certainly be valid (infact the legacy client took advantage of this). However, since Windows XP Microsoft has introduced something commonly called Session 0 isolation. Session 0 means the “root” account (or SYSTEM) is running the process (the fog client is one such Session 0 process). The main issue session 0 isolation fixes is un-authorized process elevation. Let’s pretend for a second that the fog client could infact open a UI on the user’s desktop from SYSTEM. This means that your regular old user now has access to a UI that has unrestricted access. That means if the UI can be exploited, a regular old user could do some massive damage to your system. This is why snapins are prohibited from showing any kind of UI.
The client works around this Session 0 isolation by having “user agents” run. That is, whenever a user logs in a FOGUserService is spawned in the user’s session. Then the Session 0 service communicates with the user agent as basically a relay. Need to spawn a shutdown prompt? Send a request to every user agent running to do it on our behalf. So sending a message to the user desktop is certainly possible, but at this point we aren’t adding new features.
So with that said, we could certainly display some message on a logged in user’s screen. But for the login screen? That would need some heavy integration with the Windows interactive Logon Architecture, which requires a massive amount of work, or for the Session 0 isolation to not be present.