Ok, looks like it may be possible to use now, and there's also probably some work in PySide to make this nice and convenient to enable.
The logic to define QQmlDebuggingEnabler is present (if QT_CONFIG(qml_debug) is set at Qt compile time, which I think in most cases it is), but the creation of the static QQmlDebuggingEnabler object only occurs if QT_QML_DEBUG or QT_QML_DEBUG_NO_WARNING preprocessor macro is defined in user code.
So, I think to use it in a user python app right now (if QQmlDebuggingEnabler class is wrapped by shiboken) is to construct a global variable that holds the call to qQmlEnableDebuggingHelper(bool) prior to the creation of a QQmlEngine. Since this occurs in a header file that I think is pulled in when the header for the QML engine is pulled in, I think this will happen before a QApplication is instantiated, but I'm not sure.