as far as I know there is no "customs parking" or a similar facility. I had to move my car out of the South African Custom Union (to Moz, Sim, etc) to get my CdP cleared (state 2004).
But there is the possibility to prolong the existing CdP ONCE at th SA customs (even if they charge you some Rand for that) for another second year -without the need to leave the country. Maybe this is an option for you, just to change the Carnet some month later...
But, sorry for the question, why do you need a NEW CdP? As long as no important issue had changed (eg owner of the car..) the "new CdP" is just a sequel of the older one and can easily change while leaving the country...
Greets to all "transafricans"