If you want to tap off some HV, probably the best place to do this in on the HV test point. However, there are a few things to know.
The HV generator is software controlled and is pretty closely trimmed to the management of the tubes. It only runs when the tubes are on, and so is already providing a dimmed output, even at the HV test point.
The onboard HV generator is not able to provide much more current than is needed for the tubes, at least not without a good heat sinking set up, and perhaps a bit more input voltage. However, if you are talking about a few neons with high value ballast resistors, you are going to be alright, I think.
Driving the neons for the separators is a harder job than just tapping off some HV, because the separator neons are not lit permanently, and they cause the HV generator to modulate (you can see the tubes getting dimmer and brighter in sympathy to the separators), but just adding a little extra load might not be a problem.
If you do add a little more load, try doing a factory reset so that the HV generator can be re-calibrated again.