Chaff wouldn't be a fixed object: released from a moving car in response to a tailgater, the chaff would be moving at the same speed as that car, though it would presumably slow down rapidly in the air. Depending on how sensitive the tailgater's radar is, it might appear as if the followed car is braking rapidly.
However, I'm not sure it'd be effective: the whole problem with tailgaters is that they follow too closely, so there might not be enough space for the chaff to decelerate and trigger the tailgater's radar. It would be an interesting test, though. But I think an active radar emitter that detects the tailgater's radar and generates a matching signal that appears like a rapidly-braking car would be better, though obviously more technically difficult.
However, I'm not sure it'd be effective: the whole problem with tailgaters is that they follow too closely, so there might not be enough space for the chaff to decelerate and trigger the tailgater's radar. It would be an interesting test, though. But I think an active radar emitter that detects the tailgater's radar and generates a matching signal that appears like a rapidly-braking car would be better, though obviously more technically difficult.