To share my experience, I've finally locked in the last flight for my OWA in J yesterday. Booked most legs online, then I called to add in the last two flights as they became available. On both occasions had capable (offshore) agents who were able to see the same availability as me and add flights to an existing booking without trouble.
I was worried about how I would go adding the HND-SYD leg, seeing as these seats seem to be snapped up within about 30 seconds of being released online - but the agent was amazing and after I gave her all the flight details she was happy to wait a few minutes until exactly 12pm AEDT (when JAL release seats) to refresh her screen and locked in the seats within seconds! I did need to email the rewards escalation email to get the new e-ticket sent, but good to go now
Full itinerary is below, I'll continue to monitor for a QF release of SYD-JNB seats - but happy with what we've managed to get!
QF SYD-PER
Surface Sector Perth to Cape Town (have booked SAA J for PER-JNB with Velocity points)
BA CPT-LHR
AY MUC-HEL-RVN
AY RVN-HEL-ARN (in Y)
JL FRA-NRT
JL HND-SYD
Now fingers crossed that schedule changes don't cause havoc to our plans!