From the classic Maintenance Manual, what the exact emergencies are that require the pump supply off (apart from a fire) I'm not sure of suffice to say if an a/c landed in that config it would more than likely be a pump change, or what was left of it anyway!
Operation of the engine-driven pump is controlled by the operating mode selected by the position of the ENG PUMP switch located on the flight engineer's panel.
With the switch positioned to NORMAL, the depressurizing valve is de-energized and the pump will supply fluid flow (proportional to the operating speed of the engine) as demanded by the system (Fig. 5).
When the switch is positioned to DEPR, the depressurizing valve is energized and the engine-driven pump is blocked from supplying flow to the system and output pump pressure is reduced to 800-1000 psi.
The only output is the case drain flow required for pump cooling and lubrication.
Placing the ENG PUMP switch to SUPPLY OFF or placing the ENG FIRE switch on the pilots' overhead panel to FIRE position, will close the supply (fire) shutoff valve and shut off all fluid flow to the engine-driven pump and depressurize the pump.
Except for certain emergency conditions, the ENG PUMP switch should never be placed to the SUPPLY OFF position or the ENG FIRE switch to the FIRE position with the engine operating.