You can create a QLineEdit in a toolbar (manually add it via addWidget()). However, afterwards the visible properties isn't honored anylonger and the widget is painted anyway.

The attached minimalistic project demonstrates that. 

Since the use of widgets inside toolbars is somewhat a hack by itself, I don't think this has been tested before. However, adding a "is visible" check in the QToolBar's paint routine might be easy to add.