Since there is no coordination among the various applications, you're basically stuck playing a game of walls and ladders, hoping that your ladder is taller than everybody else's wall. The customer has pretty much found the tallest ladder which the window manager provides. There is no "super topmost" flag.
Sure, you can try moving to another level of the system, like say creating a custom desktop, but all that does is give you a taller ladder. And then one of the other applications is going to say, "I need to display a store-wide page (manager to the deli please, manager to the deli), overriding all other messages, even if it's a limited-time in-store promotion." And they'll try something nastier, like enumerating all the windows in the system and calling ShowWindow(SW_HIDE).
And then another application will say, "I need to display an important store-wide security announcement (Will the owner of a white Honda Civic, license plate 037-MSN, please return to your vehicle), overriding all other messages, even if it's a store-wide page." And it'll try something nastier, like setting their program as the screen saver, disabling the mouse and keyboard devices, and then invoking the screen saver on the secure desktop.
Link thanks to andrewducker.
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