According to
this post you are breaking the rules.
SYD-PER-CBR,SYD would work, but then it'd be another open jaw, that, with the new rules will put you at 21 segments
Going SIN-NRT-HKG-JNB would free up a segment, but I cant recall whether the JAL SIN-NRT flights are two, or three class - either way its 120 tier points as opposed to 60 for the SIN-HKG - and more miles?!
EDIT: Is finishing in JNB allowed? Or is it one of the perks of entering via Asia?