How about doing a search at the alpine or rock garden forum? They probably know about that type of location. Something like sedums, sempervivums. Some arborvitae are sensitive to winter burn. Is it protected from the winter wind?
Peggy

Well, I spent a good deal of time down at Sherwin Williams today, and I am going to try stain the old cement. Going to just try a few sedums and hen-n-chicks with some alyssums, and I think that will be enough. I am getting so excited about getting rid of the ugly gray cement color, that I am think of taking a few days off just to get it done!

One perennial that I've had luck overwintering in a container on my deck is Artemisia silver brocade, also known as wormwood. I cut it back in the late fall, I don't water it, the only moisture it gets is either rain or snow, and every spring it comes right back. It drapes beautifully and might be very attractive on your retaining wall.

Artemisia is a very good idea. An even better idea since I already have some that could probably use dividing!