Your comment got me thinking about the definition of "married segments". I was under the impression that married segments worked in essentially the opposite way to what you suggest and are disadvantageous because some airlines show availability for flights through their home city if you search for them separately but will block a combination of inbound and outbound flights (with the same carrier) if you try to book them as a transit. The only work-around being to have a stopover rather than a transit. Examples are some CX combinations through HKG or QR combos through DOH. Notably, it's the operating airline which blocks the married segments rather than the partner airline you're booking the award through.
In the case under discussion of HND - SYD flights it seems that QF only shows JL award availability for that sector if you search for it in combination with an HND inbound flight as well.
Maybe someone can clarify this conundrum or maybe "married segments" should be redefined as "divorced segmants".