Yup! And I know what will grow there - these planters are the bane of quick landscaping solutions at the end of a build! Nothing seems to grow in them except weeds. Until.... I found the answer!

Try Mexican feather grass (Stipa Tennuissima). It is hardy and grows in all sorts of conditions, tolerates heat and quite a bit of drought and doesn't need that much earth. Plus they self-seed!!!
You need to get very small plugs (or divide them and plant them). In SA, you are approaching fall and winter so now is the time to get them in. May I suggest just trying a small area to see how it goes. If it works as I suspect it will, you will have cascades of green falling down your wall which moves with the wind and covers the wall. Make sure you empty all the soil in each brick and fill it with the earth of the plant (sand/soil but limited clay) it needs drainage and some watering at first.

this is it.

Looks like you need alpines. I'd go to a local botanical garden for ideas. I suspect not many of us understand the climate --- sun, heat, rain, soil. I'd bet you have so many glorious options and no worries about whether a tree will survive a -40 freeze.

Visited Hout Bay in November. The guest house retaining walls had a wide variety of succulents. Some herbs such as rosemary may grow well.

I am looking for a source of this style retaining wall in the Southeastern US. Anyone?