AFAIK, not on the 747-400. The altimeter rules the autopilot. I have never heard of 'Inertial Altitude' on that plane. There was no way the autopilot would maintain any altitude that did not come from an altimeter. GPS can give an accurate altitude, but it serves no function on an aeroplane. Any inertial derived altitude from measuring vertical acceleration is possible, but not really usefully functional in an aeroplane.