Don't forget debouncing.
In general, attaching switches to an interrupt is a Bad Idea because of the noisy nature of the input. You'll get lots of overlapping and back-to-back interrupts for each turn of the switch. Polling is actually the preferred method to handle switches.
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.