I am just guessing, but I think it would be tough to implement a protocol not supported by the base class QSslSocket. The qsslsocket_openssl.cpp code in 5.4.2 has code that appears to support TLS 1.2, but if you subclass QSslSocket, you don't have access to the class private data/functions.

Wish I had better news, but in my opinion attempting to subclass QSslSocket from Qt 4.x won't be possible.