I would be moré inclined to wrap a couple of wire turns around the fence wire
to create a transformer and get the fence pulse that way. Eliminates concerns
over fence V arcing over the R, and mitigate transients that exceed reverse LED
V spec limits. Use teflon wire, even car spark plug wire, to insure no arcing to led
circuit.
Yes on the LV side of the transformer that should be ok. Ideally you should be
developing a LV on turns around fence wire. Maybe (ignore deep math) this might
give you some clues -
There is a general topic called "joule thief" where folks are robbing small amounts (typically) of
power from various sources to power low power circuits. Worth investigating.
Neons do in fact need very little power. Modern LEDs, many visible also in uA range.
But generally you think about 1 mA or better.
While late I would look into this or find an old short thin fluorescent light tube and much like neon just place it close to the HV fence wire. The gas should ionize and done.