Aircraft have been flying since the 60's with a separated head/main gearbox i.e the Hughes 300/500 series. In the case of the 500 the gearbox is slung below the structure it drives a shaft through a hollow mast to the head which attaches to the shaft by bolts and to the mast by a nut.
In theory the main gearbox could fall out of the aircraft and the rotors and flying controls would operate normally. It is normal to remove a 500 gearbox without disturbing the rotor or flying controls.A gearbox seizure would result in the lower part of the drive shaft shearing at a shear point but the head and control would still remain functional attached to the mast.
However a failure of the mast or the thrust bearing in the head would still result in loss of the aircraft.