You can't use that space through layout because this space is already taken by the main menu and the toolbar. There is no option to put something in this space.
But you can add a main window's child without putting it in any layout. It will be shown above all layed out children. But you will have to calculate and adjust size and position of this widget manually.
For example, add the following code to the main window's constructor:
QPushButton* b = new QPushButton("TEST", this);
b->move(200, 0);
It looks like this: