How to take a nixie power supply and make a low power frequency modulated signal
Yes it is possible.  By adding one or two diodes to the standard 555 circuit, and changing one of the resistors to a pot, you can adjust the duty cycle from 1 to 99 percent -ish.  Here is a schematic grab from the innergoogle:

Probably none of the component values are correct for your application, but this shows the concept.  Also, this shows the load being switched to GND.  If you want to switch the +170 V to the load, that requires one additional transistor.  With Q1 rated for at least 300 V (safety margin), it can turn on a p-channel FET that connects the load to the +170 V.
Note:  Use the CMOS 555 - LMC555
Note:  Delete R3.
Schematic source:  https://cdn.sparkfun.com/assets/7/d/0/2/4/52433239757b7fb7798b4567.png
UPDATE:  Here is the first pass at a ((concept)) schematic.The parts values are close, but the diodes change the basic 555 equations and I have not run the new numbers.

Note:  Do not use the FET part numbers shown.  They are representative types already in my design library.  Select FETs rated for at least 200 V; 300-400 V is better.
In the upper schematic, R2 could be made variable to adjust the overall frequency, with R1 to adjust the duty cycle.  In the lower schematic, R3 adjusts the output high time, and R3 adjusts the output low time.  Two sides of the same coin.
With the resistors shown, the gate will have approx. -10 V on it.  D3 is there to protect it in case R6 opens.  R4 and R5 spread out the heat so each resistor is below 0.1 W; these should be 1/4 W through-hole parts.