Hi - I bought a new (from a dealer) car in South Africa last year. Getting it registered was really quick - from walking into the showroom to having the car registered and taxed was only 4 days. They have a strange system where you need to get the car inspected by a 3rd party to validate the VIN etc (even if it's brand new!) - but it's all quite painless. I don't know if the same inspection applies to bikes.
I'm afraid I can't help with the Carnet bit.
(04 KTM 640 Adv 'Naartjie'/11 KTM 990 Adv 'Tsumeb')