Please correct me if i'm wrong but i believe that when the 747 is coulped to the FMC via VNAV the aircraft is protected against overspeeds both in the clean and "dirty" (with flaps) configurations. The aircraft will not fly slower than the FMC calculated safe speed. This means that using VNAV the speed is limited in the approach by the flap setting since the aircraft will not reduce the speed lower than the speed calculated for the current flap setting. Again correct me if i'm wrong but i think that the flaps need to be manually selected and therefore if no flaps have been selected then the aircraft will be travelling very quickly compared to a normal approach.
Ed