Rodger Freeth drove a single seater all right - set a NZ land speed record in an Indycar on a road in the Hauraki Plains. My memory is poor, I think he set the record but I know the second run was difficult due to the bumpy road.
If we're talking riders who made the switch anywhere, not just NZ - Wayne Gardner was 500GP champ and went to V8 Supercars with his own team.
Then there is Greg Brinck - motocross, touring cars, now racing powerboats.