I've got an official confirmation that could be useful fo other FTers: the 90 minutes rule for the slow lounge only apply to passenger whose access is granted by their credit card. No time limit for eligible passengers in club/business.

Concerning the transfer itself BA EC agent contacted by the phone wasn't helpful as:
-- he had'nt any clue regarding the minimum suitable transfer time in JNB
- He couldn't indicate me if an eventual rebooking in case of missed connection could be a problem or not (even though it seems unlikely)
- he advise me to check through my luggage so that "I don't have anything to do with them, they will automatically be transferred to CPT", that just seems not true because even checked through luggage have to be cleared through customs and then drop off again in case of intl-dom connection.