Sorry, yes - good point.

You really want this DONE5 to originate in Zambia or Botswana or any of the neighbouring countries. Then the JNB-SYD can be open-dated as you have already done the first international departure (getting back to JNB). Cuts the price by a lot (haven't checked but circa 30%) as well as being more convenient. Changing a DONE5 with a sector issued in Y back to D can be an ordeal. I would consider rerouting to use CX instead JNB-HKG-SYD, it is longer but nicer and CX ticketing in JNB is excellent which might be a factor in your decision (if you stay with JNB origination).

Thanks for the suggestions. I've tried to convince my parents of this, but they will not budge. When they will start this, they will also be leaving JNB for good so will be very busy sorting out moving, taking care of our animals, sorting things out, transporting cars, tying up lose ends and so on. Despite having done this sort of thing many times, it still stresses them out immensely, and the last thing they want to do is go to Zimbabwe (especially with all the crap their beloved leader is currently orchestrating). They will eventually be ending back up in/around CPT to settle down. This also makes date flexibility difficult because once the house is empty, they have little desire to live in a hotel.

Regardless, one D seat has become available on the date they need/want to fly, so the agents have recommended splitting the ticket into two PNRs so one flies in D on the day, and another possible leaving a day or so earlier or later in D as well, or suffering in economy (probably my mother, who is more used to economy while my dad has not set off behind the curtain as long as I can remember :D).

Success
 

D availability magically opened up and now my parents will be comfortably (relatively) esconded in business class on Qantas JNB-SYD. :) Any recommendations for them, as they have never flown QF before... I read on the QF website that their aircraft will have the new Skybed. How does that compare to BA's Club seat?

Glad it worked out. Do a forum search for skybed and a few reviews and whatnot will pop up. They have all been quite favorable. Personally, I didn't like the backwards thing of the BA beds. Otherwise, they were fine.

Photos.

http://www.airliners.net/open.file/809295/M/
http://www.airliners.net/open.file/637601/M/
http://www.airliners.net/open.file/552205/M/

I will leave the comparison to the reviews here (lots of them, as Darren mentioned). Personally, when flying solo, I prefer a BA window seat, for its privacy, but when flying with a partner I prefer Qanta's upper deck.

Unfortunately I don't believe Skybed is always available between SYD-JNB. Apparently QF advertised it extensively, but then never quite delivered on their promise which is a shame when the route is one of the longest in the world. I think they are more concerned about the Kangaroo routes where there is real competition. If you use the search function you will find a a few threads on this topic. Check the configs of the aircraft and then use Seat Guru to ascertain whether it has been fitted out with Skybed.

Personally I prefer Skybed to BA's NCW because it is much longer and I don't fit that well in a bed which is 6ft. Also I find the service to be better on QF in J than on BA. I am sure your parents won't be dissapointed!

If they are in a Sky Bed equipted aircraft, ask them to try and pre-assign row 16 which is in the upper deck.