You have to subclass QNetworkAccessManager and reimplement createRequest().
I'm already doing this but in a different way.

Qt Code: 
  1. QNetworkReply *
  2. NetworkAccessManager::createRequest( Operation op, const QNetworkRequest & req, QIODevice * outgoingData)
  3. {
  4. QNetworkReply * reply = QNetworkAccessManager::createRequest(op, req, outgoingData);
  5.  
  6. 	if (checkIfAsync(req.url().toString()))
  7. 	{
  8. 		QObject::connect(reply, SIGNAL(finished()), this, SLOT(asyncFinishedHandler()));
  9. 	}
  10.  
  11. 	return reply;
  12. }
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I would like to rewrite it in such a way I wouldn't have to connect individual replies with my slot anymore. I would like to handle the difference in reply's type in one place, in handler of this signal:
Qt Code: 
  1. void QNetworkAccessManager::finished ( QNetworkReply * reply )   [signal]
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Then you can return whatever you want.
Any customer can have a car painted any colour that he wants so long as it is black. -- Henry Ford

C++ supports only* covariant return types in overridden virtual functions. (* - my addition.)

Shortly speaking you can't return whatever you want
But the question was how to subclass QNetworkReply?

Having

Qt Code: 
  1. class MyNetworkReply : public QNetworkReply
  2. {
  3.    bool myFlag;
  4. };
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how to reuse
Qt Code: 
  1. QNetworkReply * QNetworkAccessManager::createRequest ( Operation op, const QNetworkRequest & req, QIODevice * outgoingData = 0 )   [virtual protected]
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to set QNetworkReply part of MyNetworkReply object?