I don't know if reversing the entire grid is fair, but they do something similar in British Touring Cars where they have 3 races in a day. Typical races have somewhere around 26 cars. Prior to the third race a number is drawn between 6 and 12. For the third race the grid is reversed down to that number based on race 2 results. So if 10 is drawn, the first 10 cars are reversed and everyone after that starts as they finished race 2.
I think it could be good where there is a sprint race. After the sprint race, draw a number between say 7 and 12. The cars down to, and including the number drawn are reversed, and the cars after that number are also reversed. Might have to make the sprint race worth more points, so it's worthwhile dropping to 12th for the main race after winning it.
British Touring Cars also had 'success ballast' until a couple of years ago, so successful drivers had to carry more weight. Could be another option.
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