Do appeals to super selectives potentially only succeed on the proof of medical certificates that the child was ill? Would mention of distress just be dismissed?
If the child was really distressed during the exam, it's likely to be mentioned in the invigilators log - and they may well have been removed to another room to continue the exam (both for their own sake and for the rest of the children in the room).
If the child was 'just' very nervous, the panel might think that many of the children taking the exam would be in the same boat - including many who passed.
I remember waiting in the school hall for DD2 to come out of her exam - one of her friends came back earlier - pale, crying, exceptionally stressed. She'd broken down a couple of times and was removed to a separate room to continue. Come results day, she had one of the highest scores possible.