Может стоит сделать виджет прозрачным другим способом? Вот в асистенте есть такое:
windowOpacity : double
This property holds the level of opacity for the window.
The valid range of opacity is from 1.0 (completely opaque) to 0.0 (completely transparent).
By default the value of this property is 1.0.
This feature is available on Mac OS X, X11 platforms that support the Composite extension, and Windows 2000 and later.
Note that under X11 you need to have a composite manager running, and the X11 specific _NET_WM_WINDOW_OPACITY atom needs to be supported by the window manager you are using.
Access functions:
qreal windowOpacity () const
void setWindowOpacity ( qreal level )
Функции относятся к классу QWidget