I have personally hooked up directly to the disk (removed the controller that is screwed on the outside) and directly wiped out SMART parameters as well as reset logs and other such things using a serial cable directly attached. ;)
Not sure I follow how you could do this since the motor control is on the controller and is more than just adding a voltage to get it to operate, as well the servo arm which runs in a magnetic field and it's also on the controller so you would never get the heads to load. Of course I'm talking about a modern day hard drive, not one from the days of yesteryear where the heads were controlled by stepper motors. Also not sure what use a serial cable would have been since you would have had to have some electronics to talk to if a serial interface were to really work.
Out of curiosity, how did you know you wiped out the SMART data?
But we both could be right, who knows what each manufacturer does. And I don't mean to come off offensive or anything, just with what I know, it doesn't make sense.
EDIT: Found a reference to a thread (article got deleted) where the author stated that some manufacturers recorded SMART data on a -1 (negative) track thus not interfering with normal hard drive formats, and the other method was recording it on NVM in the PCB. The -1 track makes much more sense to me as I couldn't understand it any other way.