Unlike "Display", and unlike what I expected, "DisplayTopBar" displayed the message I wanted behind some GUIs. It got drawn between the background and said GUIs, which filled the screen. I wonder about this, and would like to ask either for it to behave like Display or to document this in the manual.
Also, AutoComplete let me know about DisplayMessageBar. It is undocumented, but looks like a swell idea. I hope lack of documentation doesn't mean it's about to disappear?
DisplayTopBar uses the same display layer as DisplaySpeech, so yes it will be behind GUIs.
As for DisplayMessageBar, it should work but it's a bit specialized and not something most people would use, I guess I should probably document it though.
It's something I'd like to use, for one, when DisplayTopBar (and DisplaySpeech, I guess) gets the ability to be drawn like Display. :) Not a problem right now, though.