Tried to look it up and saw provisions for changing cars and drivers (penalty is to start at end of field) but couldn’t find anything that says the driver has to be in the vehicle - seems they doubt it would be an issue for the winning car...
Lol! But, as we who have lived long enough to know, are aware, crazy and totally unpredictable things do happen. I'm sure if we gave it some thought you and I can come up with a possible scenario where a driver could somehow not be in his car as it crossed the finish line. Might take us awhile, though! :)