What might be more impressive, is re-thinking your design in such a way that you no longer need intrusive UI features like a pop up message box. Instead, try to not interupt the workflow of your user and use passive messages instead.

Note that what you propose, is already working (like in KDE).

What do you exactly mean by “passive messages” ?
Something like using a QLabel to warn the user?

A passive message is something that comes into view to the user without stopping original workflow. It could be a label or some text overlay that does not catch the mouse, or…. Think of it yourself, without stopping user interaction with a message box.

In fact, the whole point of a QMessageBox is to protect sensitive actions.
A passive message wouldn’t really forbid the user to make other actions with the application, no?

That’s the whole point. You should try to avoid forbidding the user to do something just because it is convenient for you as a programmer. Instead, make actions undoable, for instance.

Always keep in mind, that your user is trying to achieve something with your program. Your program is a tool, nothing more. That tool should support whatever the user is trying to do, and not hinder him in his way to do that. Popup boxes disrupt the flow of work, and of thought and focus of the user. They can also scare the user if you start giving warnings they may have trouble to comprehend. That hinders him in his task.

So, rather than spending time on fancy graphics effects that are really the task of the window manager to handle anyway, I would suggest rather to spend your energy on making sure the user gets done what he want to get done without being bothered or scared by your application with such dialogs.