Thanks for the tip! I booked one for 609 e. I was looking somewhere to spend about one week in March and the price for going to JNB wasn't much more than Economy Flex prices to many of the European destinations on AY right now. I don't really mind spending a few days in Kruger NP On the way back the layover in LHR is 8.5 hours on this fare, but lounge hopping in LHR T3 isn't that bad either

So regarding the topic, let me share an up-to-date experience on seat selection on BA-operated flights with AY code. In short: no improvement since 2019. I got auto-assigned seats according to my AY+ preference (window seats) on the BA flights, but wanted to change them (the JNB flights are operated on A380's and the auto-assigned seats were on the main deck, but I prefer the minicabin in the rear of the upper deck). The AY PNR opens the booking on BA MMB, where it looks like seats could be selected, but the selection doesn't stick. AY CS couldn't select the seats either and asked to contact BA. The BA Twitter team was able to provide me with the BA PNR, with which seat selection for the BA flights works on their website (I assume that AY CS could provide the BA PNR as well, if you would ask for it).