A neon will clamp to about 90V I think so if you put a lower-voltage device (LED in opto isolator) in parallel with the neon, the neon will probably no longer light once the LED takes the current.
Some opto-isolators have very high current gain, like 5x or more. They should allow you to detect a mA or so with no trouble, particularly if you load up their output side with about 20k. Even a 6N138 should work OK with 10k load to reliably detect 0.5mA with >5V swing.
Note that because of the diode input, you will get an output at 50/60Hz with a bit under 50% duty cycle so the software needs to be aware of that.