bump... can anyone point me in the right direction?
Without knowing the specifics of your particular neon bulbs, here's my take on it.
I'm not sure how much excess current that supply has above and beyond the Nixie requirements, but surely it can spare a milliampere or two.
Remembering the little old NE-2 bulbs that we used to use before LEDs became practical, here's what I would try:
The bulbs themselves had a DC strike voltage of about 90 Volts, and "normal" illumination was attained somewhere between 0.5 mA and 1 mA. (The power was somewhere between 1/16 Watt and 1/25 Watt, depending on specific tube type.) After the bulb was illuminated, the voltage dropped to something like 60 Volts, as I recall---maybe a little lower---this is temperature-dependent.
I can't imagine that a neon bulb would do something horribly dramatic if you put a milliampere through it (maybe a little pop, but nothing earthshaking). I'm assuming you have a few "spare" bulbs to test with.
If you try a series resistor of something like 180000 Ohms (that's 180 K Ohms) to the 170 VDC power supply the operating current comes out something like
(170 - 60) / 180000 or a little more than 0.6 mA (Off the top of my head).
If this isn't bright enough, try 150K or lower. If it's too bright, try 220K or higher. (Change resistor values something like 10%-20% between trials.)
If you know what the operating voltage really is, pick a resistor to begin with that gives you something between 0.5 mA and 1 mA operating current. If you have a larger bulb that takes more current, you can try successively smaller values of resistor.
1/4 watt resistors are fine for these tests.