Answering my own question. Adding the org.freedesktop.DBus.Properties interface to my xml did not work, as the QDbusAbstractorAdaptor or anyone else is already implementing theses methods. But the signal will not be emitted. At least I did not succeed in finding a "official" way.

But I found a workaround which work for me: https://randomguy3.wordpress.com/2010/09/07/the-magic-of-qtdbus-and-the-propertychanged-signal/

My Adaptor parent class is using the setProperty and property functions of QObject.
Overloaded the setProperty function, calling the QOBject ones and as an addition emitted the PropertiesChanged signal manually like this:

    QDBusMessage signal = QDBusMessage::createSignal(
    "/my/object/path",
    "org.freedesktop.DBus.Properties",
    "PropertiesChanged");
    signal << "my.inter.face";

    QVariantMap changedProps;
    changedProps.insert(thePropertyName, thePropertyValue);
    signal << changedProps;

    QStringList invalidatedProps;
    signal << invalidatedProps;

    QDBusConnection::systemBus().send(signal);

Not a very nice way, but at least the signal is emitted.
Anyway I would be interessted in an more official way of doing it....

Cheers
Thilo