How to send a signal over long wire that is pulsing 9kV DC (agricultural electric fence)
Don't know specifically how this unit works, but I could propose how I might solve the problem!

Hand unit

Put a magnet coil inside the tip of the hand unit. Monitor the coil so you can detect when the high voltage pulse happens. 

After you see the pulse, you know you have an open transmit window before the next high voltage pulse. During that pause, transmit a sine wave onto the wire via magnetic coupling through the internal coil. The best frequency will likely be in the 10's or 100's of kilohertz. Bonus points if you add a capacitor to make the whole loop into a giant tank and have the hand unit transmit on the resonant frequency. 

Fence driver unit

Back at the fence driver unit, in between the high voltage pulses you would connect the two ends of the loop to a transceiver. This could be a transformer if you want to look for current, or something like an op-amp if you want to look for voltage (current probably more immune to noise?). Use a resonant tone detector to sense if the hand unit is transmitting. With careful design, you can detect the tone at astonishingly low levels. 

You can modulate the tone, or use multiple tones, to transmit actual data with checksums to avoid false positives from noise on the wire. 

Note that you want lots of serious protection on the ends of the big loop for things like transients and lightning! That is a very big loop so it it possible for some very bold things to happen! That would also be a great loop antenna for communicating with aliens! :)