On the using AA flight number for JNB/CPT-LHR segments, this is obviously of increased interest since the new rule of no elite level bonus AAdvantage mileage when traveling on BA except when on an AA codeshare flight number. SABRE will only allow booking the AA codeshare number on JNB/CPT-LHR segments (both directions, of course) when connecting to (or from) AA metal. I traveled this a few days ago, LAX-LHR-JNB. Returning to LHR and then onward to HKG, however, would not allow AA codeshare.

However, if you have direct access to SABRE (which I do not, but once did), my guess is that the following might work: Book, for example, JNB-LAX connecting at LHR using AA numbers. If this shows in the PNR as two separate segments, then you should be able to cancel the LHR-LAX segment, and be left with JNB-LHR standalone with an AA number.

I'll be curious to hear reports on this.