Correct me if am getting it wrong -
You get a pointer to the object in the filter. Cast it to a widget using qobject_cast and ask for the tooltip using QWidget::toolTip().
I had installed filter on qApp. Wont the pointer to the object be of QApplication type ??

I had tried QApplication::activeWindow and QApplication::focusWidget. But I was not getting the proper pointer. In QApplication::focusWidget I was able to get a pointer to widget, which was of QPushButton type. But I needed a pointer to the action