Update: I found someone on Discord who unlocked it by happenstance. You have to finish a Grand Prix race with the lowest possible Safe rank provided the number of racers doesn’t drop below it.
So for example, if the Safe rank bottoms out at 40 to qualify for the next race, you must finish in 40th for it to unlock. If there’s 39 racers or less left in the race and you place last (which technically should meet the criteria), it won’t unlock.
I was hoping this wouldn’t be the case, because virtually every race has enough racers KO’ed to drop below the rank. This is so needlessly situational, and I dislike how it encourages sandbagging.