Possibly work-around: You could initially limit the quiz to 1 attempt per student, then when you manually grade a student's attempt, add a user override to allow that student one more attempt.
Ok thanks for the feedback Tim. We have used 1 attempt before for self-marking quizzes to make sure students don't use up all their attempts before asking teachers about where they went wrong. We might look into making a plugin but that will go on the end of the to-do list, which is always long.