Don't forget debouncing.
You could try to do something externally with a comparator and a latch, but it would probably be more complicated than it's worth. Use a pull-down resistor so that the output goes to gnd when the wiper is not connected to a pole, and set the comparator to signal when its input goes lower than the lowest value of the switch. Feed that into a latch, and the output of that can go to an interrupt pin. You'd need to reset the latch after each read, though.
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