Thanks for all of your replies. I tried calling SAA for my original booking through UA, and they could only assign JNB-LHR, not the IAD-JNB flight. One option is certainly to try booking again through UA, and try calling SAA.

But given the other comments above (and the fact that my spouse is willing to forgo the PQDs he needs for silver status), I may just book directly with SAA. If I do it over the phone, I might be able to get a booking class that will get me 100% PQMs.