Cecco
Once the aircraft is in SA on a General Declation (and within the first seven days) write a letter to Customs at the airport of entry detailing the requirement. They will then issue an extension. Make sure you leave within the period though because then you are on the radar and toast if you overrun.
If the aircraft enters SA and you move it to a non-entry airport the problem is less severe. The system for checking is a rather crude one. The apes drive around the airport regularly and write down foreign registartions. Non-entry airports have no officials and therefore no register is taken.
Somehow their trillion buck computer system appears to not have this functionality built into it.
Lanseria is by far the worst airport for this. Polokwane for eg. has officials that are a lot less unpleasant. I believe the clowns here are paid a commission for issuing penalties so they are exceedingly quick to get completely unreasoble.