Would a 100KΩ resistor connected to ground pull an open analog line down to 0
With a 100 kΩ resistor to GND, it is not guaranteed that the value will be zero. If a (servo)motor is connected, it might get some noise.

The pull down resistor can be any value, because it is possible to calculate the value of the photoresistor.

There is an alternative solution. I think that all the analog pins of the Arduino Mega 2560 are digital pins as well. You can do a pinMode(A0,INPUT_PULLUP); to pull the input high. When a photoresistor is connected, you can turn the internal pullup resistor off with pinMode(A0,INPUT);.