Originally Posted by
anabolism
...
I haven't tried ITA, but EF requires it be a connection, and I've had problems booking even overnight connections as codeshares. I'd be surprised if AA let you ticket as a codeshare with a stop. As for the fuel surcharge difference, I was under the impression that AA now charges the full BA amount. Has that changed back?
Despite its eclectic UI, I find ITA to be far more reliable in determining availability and obtaining detailed pricing for a planned trip. (I do maintain a subscription to EF for other uses, though)
I asked ITA for A space JNB-LAX for next Wednesday using the qualifying line
AA+ X:LHR /f bc=A
and got a bunch of flights - probably all the chronologically-possible combinations - and have zero doubt that they're bookable. The fare breakdown included USD220 for AA's YR on the trip; my recollection is that BA's YQ is a couple hundred dollars more, but you can find out for sure simply by changing the AA+ to BA+.
As far as allowing the codeshare with a stop in London, I don't recall why I opined that it was okay, but you can try that too - just use the multisegment input. Given the huge difference in Passenger Duty if you stop, you do need a good reason for it.