A normal mains switch can be much cheaper and you could have a light to show when its on
Depends on the voltage output of the electric fence charger.
Given the ranch systems I've run in the past, arc-over in the switch to ground in damp weather is probably a given and so I'd never have considered it. And unless that's a neon light with a resistor that can take the voltage and limit the current properly, it will soon go bye-bye.
If you don't mind getting zapped, you might try it .
There's a reason that electric fence equipment isn't cheaper.
Note: Question has been changed. Originally sounded the OP wanted to use it as the high voltage disconnect.