Just called AP to look into this...
YVR-JNB 16158 (quoted)
YVR-CPT 16108 (quoted)
Both of those well exceed the Expertflyer values YVR-JNB there is listed at 12412 (atlantic) 15070 (pacific) and YVR-CPT 12813 (atlantic only).
So I followed up by asking her does that mean as long as I stay below 16158 it doesn't matter which way I cross? She didn't really answer other than by saying they have to validate the route, so take that FWIW. But those can both be added to the sticky. I'm quite pleased that the agents knew exactly what I was talking about and answered, the whole thing took less time than getting through the menus.