Not sure what "BA Amex PP" is, eiflyer, but there's a SLOW Lounge in JNB International departures which can be used by certain credit card holders regardless of flight, and is used by BA Status pax, but I think you have to be on a BA flight, so chances are you won't have access, but no harm in checking it out.

I got an FNB Commercial Platinum card for just two of the perks: free SLOW Lounge access and free travel insurance. At 4$ a month a no-brainer really.