it has no effect though...
Of course not. A QAction is an abstraction of a command, not a user interface element. It holds properties (like text or icons) that can be used by actual GUI elements (like toolbar buttons or menu items) to render themselves, but the QAction itself is just class to hold all these things in a convenient place. Giving it an object name doesn't give it magical powers.

If the QToolBar class itself has no way to change the properties of individual separators, then you are stuck unless you want to write your own toolbar class derived from QToolBar. You will have to rewrite the paintEvent(), in particular the part that paints the separators. It probably will not be easy.