The problem is that GUI stuff is all client-side, so when you say that it makes it mount for everyone, what it really is doing is just showing for you, and nobody else.
A working solution would involve a client add-on with a clientCmd to show or hide it, and the server sends a commandToClient to tell the client when to show it or hide it.
Just imagine if someone made a horrible image (think of the worst thing you can think of), and created a crosshair with it, full screen, and made it show up for everyone on the server... That is (one good reason) why it can't be done with just a server add-on.