I looked at the movie, second-by-second, to weed out the BS...

If you notice, he makes a hard turn right into the wall right at the ramp... if he kept this turn throughout the roll, at a given speed AND with a very carefully calculated length of the 'loop', it just might be possible. Constantly turning into the loop could keep the G's in the right direction, just long enough to keep him planted. If the total length of the corkscrew was any longer, he would most-likely fall, because the acceleration of gravity would exceed the lateral G's pushing the car into the roll. It seems that just before the point at which gravity would win the battle, the car has passed the upside-down section and is returning to a "tires-down" direction. Just like when we humans jump into the air -- there's the initial energy that we exert to surpass the pull of gravity, and then the top of the jump where our force EQUALS gravity, with gravity finally winning.

It may still be wonderful editing, but I can't help but exert wishful thinking, hoping that it was indeed possible.