I want to power up a little flash camera as an indicator like neon indicators, so I can get a little while glow. I just want to feed it with a couple of 10s or 100s of microamps that should be enough to get any glow, but I don't know what resistor to use.

In a camera, an ~120uF 350V capacitor is connected peramanently across the main electrodes of the lamp. The capacitor is charged to near its full rating (330ish V), but this does not suffice to strike the lamp

When the photo is taken (so in sync with the camera exposing the film or digital sensor), an ignition pulse of few kV is applied between one electrode and an external probe on the flash tube. In some flash units the external probe is a wire wound on the outside of the tube, in others the reflector. This starts a breakdown in the lamp, which results in ionization and the tube short circuits the charged capacitor, turning all its stored energy into a flash. (the capacitor remaining voltage is few 10's V at most after the flash)

So :

1. You need a source supplying higher voltage. 220V won't strike the lamp

2. for really continuous light and not series of flashes, you need a source which behaves as real current source across all voltages up to the lamp striking voltage, or at least a pulse train (a source that outputs continuous sequence of precisely uniform pulses) for fast flicker that could be considered as a constant output. For example, using a high voltage CCFL inverter or something similar. Running the lamp on something that has capacitance (like a voltage multiplier) will result in the capacitance charging up to the tube striking voltage in a loop, so make a series of flashes instead of continuous light

Considering the voltage across the tube is very high vs. the tube size, you must supply very low current to not overheat it

I want to power up a a tiny 2cm flash from disposable cameras and digital cameras, which only have two pins and not a probe. I wonder if connecting it to 220V with a 100K resistor just like a neon indicator would work. I don't want to light it up at a full brightness, just a little glow. I don't think that more than 220v is needed for make a little glow, I have an old fluorescent tube that glows a tiny bit just with the ballast in series with the starter removed. I think that anything from 1M to 100k resistor in series might work.

I want to power up a a tiny 2cm flash from disposable cameras and digital cameras, which only have two pins and not a probe. I wonder if connecting it to 220V with a 100K resistor just like a neon indicator would work. I don't want to light it up at a full brightness, just a little glow. I don't think that more than 220v is needed for make a little glow, I have an old fluorescent tube that glows a tiny bit just with the ballast in series with the starter removed. I think that anything from 1M to 100k resistor in series might work.

It wont work, because the ignition voltage is way above the 330V. The fact the tube is shorter does not change anything, as the gas fill is intentionally designed so the lamp wont ignite without the HV pulse (so higher Xe fill pressure). The reason is to maintain the characteristics of the arc, namely color and efficacy. And that needs to maintain the voltage levels, as well as an arc power density the same as on "bigger" lamps.
Plus there was the intention to share as much as possible of the ballast components (so having pretty much the same for all sizes, except for the components directly responsible for the energy, so the main capacitor and its charging circuitry)

Is 2kv at 10 microamps enough for a constant operation? The lamp should have a 0.02 watt of power dissipation, which should make the lamp last practically forever. So I'll need a voltage multiplier, but I want it to be constant brightness and not flashing, but adding an electronic smoothing capacitor will not last much and it will leak, I want to make my lamps as long life as possible. I hope that the lamp doesn't become too flashy at some minimal voltage that the flash can take of the multiplier iterations. Because I'm not good with the maths and formulas I'll need your help to choose the correct capacitors and diodes for give some usable voltage at 10 microamps to the flash.

The 10uA will charge the wiring capacitance, till it reaches the breakdown voltage. Then the discharge initiates, forcing the voltage down to some 100..200V, where the discharge probably extinguishes. So as result the lamp would be just flashing wildly.
Dont forget the lamps are designed for high pulsed current operation, the low current will be deep in the steep negative dynamic resistance area, so practically impossible to maintain a stable discharge.