Where are you heading?
I found bike hire difficult in Durban but hooked up with Roger at the BMW off road school there and had an ace weekend. He could organise a great few days for you either into Lesotho or off road to the battle fields.
I hired in cape Town and was able to ride the garden route for 8 days with route plans and hotels booked ahead. A bit more expensive (albeit cheep by UK standards) but the hotels were excellent and the food beyond historic - I'll get out details if you are interested.
If you want to ride from Johannesburg, may the Lord have mercy on you!
Namibia is excellent and there are some hire firms there.
cheers
Simon