It's still possible with some rather ugly "hacking". One would need to use for example
QObject::findChild() to get access to the private corner button. An event filter is required for "overriding" the button's paintEvent() (QTableCornerButton::paintEvent() doesn't paint the text even if the button had text set). I have attached an example.
This is, of course, not recommended way for doing it. It is a private button and may be a subject to change in the future. I suggest sending a suggestion about the issue to the
Task-Tracker.