Does this switch exist? Answered
It may just be my google-fu failing me, but I can't find switch I am sure must exist:
A sliding switch with reversing (DC) polarity.
At one end of the slider, power is full "forward". Slide the controller, and power decreases. Reach the centre, and power is zero/off. Slide past the centre, and power increases again, but with reversed polarity, ie in reverse.