Why would any pilot want to take an aircraft beyond it's defined envelope?
During acceptence flights, I've taken the Classic to Vmo/Mmo, 392 KIAS/0.92 M. Between FL 26 and 29 in level flight advance to maximum rated thrust and accellerate. This usually produces something about 380KIAS. A slight push over will result in the 392/.92 rather quickly, with the over speed warning within 3 kts and .01 M. And thats what we want to see. Every once and a while you get a very slight burble or low rumbel as the air starts to go supersonic somewhere above the flightdeck.
Generally the intake airspeed of a jet engine does not like supersonic airflow. I would hate to see the dampage on the N1 rotors after any lengthy exposure to this rate of airflow and the boundary separation that would be occurring. As the airflow entered the N2 the engine would likely fail.
Want to go supersonic in a 747, go fly down wind in the jet stream, as previouly suggested.