viewer-centered perspective and hand-tracked steering

[NaN]

but "injecting" the code for the marker-tracking based steering seems less trivial

The most simple way would be to override carinputs[CARINPUT::STEER_LEFT] and carinputs[CARINPUT::STEER_RIGHT] just before car.HandleInputs(carinputs, TickPeriod()); in void GAME::UpdateCarInputs(CAR & car). The value range is [0, 1].

A more sophisticated approach might be to hook your code as a JOYSTICK to EVENTSYSTEM_SDL. And add your input polling code to void EVENTSYSTEM_SDL:rocessEvents().