Well, if it's an unrated class, like those "go clear within the time and get a blue ribbon" schooling classes are, they can do whatever they want and just score it as II.2.b-which is simply combining jumping and time faults.
I don't think there is any requirement for a jump off between those with equal faults in non rated, basically non competitive class where there is no one winner. Or the show to do anything other then post the course and state what table they will be using to score it in an unrated schooling class.
But they certainly can include a jump off for riders that are clean as a requirement for that blue-a double clear if you will. You just need to ask at the show.
When opportunity knocks it's wearing overalls and looks like work.