The description of the African carry had me really confused, because I thought the sling went over the head. This picture helped sort thing out:
When I sling my gun, I put my head and left arm (reaction side) through the sling, so that the sling rests on my right shoulder, and my gun would hang on the left side. If I'm going to be carrying it, I'll try get it snug against my back, but that doesn't happen very often.
As for the pistol holster, I keep it on my strong side hip at the belt, or just below my hip with a drop leg, depending on which vest I'm wearing. I'm imagining that trying to cross draw from your reaction side hip would be a pain in the ass, and would slow you down. If you add to it that you're trying to find a place to swing your primary to, it's even worse. If the pistol is on your strong side, you can toss your primary to your reaction side, to get it out of the way. If you do that with a cross draw to your other hip, you have to toss your gun really hard so that it swings behind you. Otherwise you have to drop the gun straight down (or to your primary side), and reach your arm OVER your gun (in the sense that your strong hand has to cross your chest, and move away from your body OVER the gun) to get to your pistol.
If you're going to be in a position that makes it difficult to draw from your hip (ie a sitting position in a vehicle), I think it would make more sense to have your crossdraw holster placed on the front of your vest, and not all the way on the opposite side.