Can you get a Doctor's letter in advance? Is there any medication that you can ostentatiously take? (I'm serious!)
Take a walking stick, one that is well used, perhaps borrow from a friend? Or take some form of walking aid to which you are entitled (or might be!). In other words, make your internal problem - externally visible!
For DVT, consult your doctor but an Aspirin 12 hours before travel and again at departure is recommended by many.
With AF, their web site shows Airbus 321 to CDG for the change onto a 747-400 to JNB. On the short sector, don't waste time trying to get moved. It is a short sector and no chance to build up understanding with crew.
I hope you enjoy South Africa, I grew up there.
