I don't think the resistor is the correct item to control the 70 volt problem. I would use a transistor to turn on the lights. Just wire up the transistor as a switch. The ground of the Nixie has no voltage until the light is lit. Use this ground voltage of the nixie to turn on, or open, the transistor to the other light. Then when the nixies are off all lights will be off. And when the nixies are on there is voltage going to ground that turns on the transistor and the other lights turn on. I hope that is clear enough. Lets see if I can do some ascii art here.
Nixie ground pin
0------|-----------------------ground
|
|----------------------base of transistor
Figure out the rest.
Now I have never worked with nixie tubes. So there might be enough voltage flowing to ground when they are off to open the transistor. I just don't know. But a little work with a VOM will tell you if this idea will work.