If the voltage dropping thing idea really was for dropping the supply voltage and not for putting a divider to the temperature measurement to fool the camera, then a proper regulator is the only way to go.
E.g. for my A570IS (a dual AA cell cam) the nominal required external PSU output is 3,15 V, 3 A if my memory serves. That's almost 10 W to dissipate in PLAY mode or camera OFF mode if you go with a zener (you don't, really).
A resistive divider is not possible either, for the same reasons and more.
Whatever you do, if you use too high impedance in your external PSU and you manage to power on, your camera will shut down when it operates a motor due to high current causing a voltage drop and brown-out protection shuts the cam down.
