Airclues:
I was involved in several of these tests, and carried out many full stalls on the 747. On no occasion did I see the slightest hint of a wing drop during the stall or recovery. The ailerons were effective right down to the stall. We always kept our feet away from the rudder pedals as we were told that any rudder input would cause a rapid roll.
Thank you. Thanks also to CapnBloggs, WhyBy, and CaptAirProx. With your points in mind, I have watched the sequence of events a few more times. Makes more sense now. Got a better picture of the yaw with the right roll. Also paid more attention to the ground impact and the inertia/forward splash of debris.
Much obliged, gentlemen.
*tips cap*
Enter guesswork here: with Airclues' point in mind, and considering pilots trying to make the aircraft respond (this don't work, that don't work, the other don't work, try this ... ) some chance of a right rudder input made (in desperation ... never give up ... try to make it fly ... get nose down) thus the right wing drop.
And maybe not.