Togabutton, long time no hear....how's the island?
My experience is as above, but a few things to keep in mind.
Even though you do not need to get a ppl issued to take the flight test, you still have to go through the procedure of getting your license verified by the South African CAA.
Fill out the forms, submit them, and the FAA will get back to you with a letter confirming the validity of your license. You will be asked for this by the examiner.
ALL ATPS flight test is straight forward. Oral portion consists of describing the systems of the seminole, no more, no less. Flight portion is upper air work (steep turns/arrival and departure stalls/single engine work) and then multiple approaches. I ended up doing ILS/VOR/NDB/GPS approaches, some single engine, some with circle to land.
You get an ATP MEL.
For the single engine add-on I just did the comm single engine vfr test in a C177 at the local flight school. Basically like a RSA ppl test without the cross-country.
What you will end up with is an ATP MEL with Comm single engine priviliges. Unless they design some monster single engine aircraft weighing more than 12500lbs, that should see you right for most applications.
Good luck.