I had the same issue when booking a BA redemption on JL. My record locator was not recognised by JL system.

When I added the booking to the MyFlights app, I noticed that a second record locator was mentionned. I used this one on the JL EU page and was able to select my seat (was not able on the JL Japan page).

I guess you can try the same, and use the Aussie page or the EU one.