Neons have a trigger voltage of around 90 volts or so and were once used as a form of constant voltage source, rather like zener diodes are currently used, so they'd be no good at providing an analogue value.
However you could build a simple potential divider system using high voltage resistors such as available herehttp://www.willow.co.uk/html/high_voltage.html
Assuming you want a drop from say 20,000 volts to 5 volts (to allow for the pulsar giving more than 8Kv) you would need a division of 4,000:1 (OK not exactly but near enough) with say 1000 ohms as the input or sense resistance, you'd therefore need 4000 x 1000 or 4 megohms as the main dropper resistor. To ensure against track-over this could be made from 4 off 1meg resistors each rated at 10kv wired in series.
In effect you'd end up with a chain of 5 resistors, the lowest of which provides the ground and +5 volt sense to be input into the arduino.
Also place a diode across the 1k ohm detection resistor reverse biased to ensure you only detected a positive going pulse.
Cheap and cheerful with a known calibration factor.