I'm not sure quite what your priorities are.

First, if you simply want to fly in the pointy end (Zone A) of a 747, you can still do that on Qantas aircraft. I was in row 1 about a year ago on JNB-SYD, which is in the same place as row 1 on a BA 747. It's business class on QF, not first class, but it's still the pointy end.

If you specifically want to fly in first class, then that's a different story.

Second, how important is it that you can fly QF to get to the BA flight? And if you're prepared to go to JNB to take BA to LHR, would you rule out simply coming to London and taking your pick of all of BA's 747 routes? After all, JFK is one that comes to mind as a route which quite reliably has some 747s all year round.

Third, what exactly is your criterion about earning QF status credits (which you say rules out DXB)?

FWIW, off the top of my head the current pattern on JNB has recently been that during the northern summer it's a 380/747 pair, but during the northern winter BA is operating a double-daily 380 (with additional non-daily 787s starting with the forthcoming NW season). So depending on when you want to do this, JNB would be a viable alternative in addition to CPT.