You are in luck! Many varieties of succulents should do very well in your situation. And, there are so many to choose from, you should be able to find several that you like. Many of them will hang down the wall, like you suggest and may also root at the base of the wall, when pieces of the plant accidentally fall off and land there. Also, have you tried portulaca? In some of the bigger spots where there is a little more room for soil, those might work and they are quite colorful. What about small juniper specimens like procumbens or even something a little bigger like blue rug? You might have to take a one-gallon plant and "divide" it a little to get a plant small enough to push into the spaces, but once they get going, they cascade beautifully. I am not sure about climbers, because it's not my forte', but I'll bet others on the forum have some good suggestions.

I meant "many varieties of succulents, such as sedums."

I have phlox subulata, hens & chicks, dianthus "Firewitch" that all do well in a similar situation - even a few ostrich ferns that stay small and "cute" in tight cracks and crevices. As mentioned, there are quite a few kinds of creeping sedums. Birds nest spruce might work too - in 20 years it could reach 2x3' in size. And one gallon size BNS's are @$6 at the Home Depot, up here anyway.

Is there any way you could provide some kind of temporary shade while new plants are settling in? A piece of burlap or cheesecloth suspended 12-18" overhead on bamboo stakes might do the trick.

A little late in your question, but if you can purchase portulaca or moss rose, I think you will enjoy the colour. Needs full sun and is drought resistant.

I would add one more item to the list of possibilities - ornamental grasses. There are some smaller ones that look incredible nestled between rocks. They are drought tolerant and take the heat well. The blue fescues are especially stunning with rocks.

Regards,

K4

You can also use thyme. There are many varieties and they dont like to be wet. THere are quite a few plants the like dry conditions. I have the opposite conditions in a lot of my garden, its very wet here and some plants dont like it. I have to add sand around the plant if I want to plant dry condition plants.
I love the sedums and sempervivum (hens and chicks.)
heres a list of drought tolerant plants,you will have to check them to your zone.
spring
aurinia saxatillis
Suphoribia polychoromia (cushion flower)
Pulsatilla vulgaris( pasque flower)

summer
Acanthus mollis( spiny bear breeches)
achillea spp. (yarrows)
Anthemis tinctoria (golden marguerite)
Ameria Maritima(sea pink, thrift)
aclepias tubersoa(butterfly weed)
Belamcanda chinenis(blackberry lilly)
Calaminth nepeta(calamint)
Cerastium tomentosum(snow in summer)
Coreopsis verticllata (threadleaf coreposis)
delosperma cooperi (hardy iceplant)
Gypsophila spp.(babys breath)
Iris hybrids(bearded Iris)
Lavandula angustifolia(lavendar)
Limonium latfolium(sea lavendar)
Nepeta spp. (catmints)
Oenothera berlandieri(showy evening primrose)
Santolina chamaecyparissus (lavendar cotton)
Saponaria ocymoides (rock soapwort)
Scutellaria baicalensis(skullcap)
Thymus spp. (thymes)
Verbena canadenisi (clump verbena)
Yucca fliamentosa (adams needle)

late summer & fall
Echinacea purea (purple coneflower)
Ecgubops ritro (globe thistle)
Eryngium amethystinum (amethyst sea holly)
gaura lindheimeri (white gaura)
Helianthus angustifolius (fall sunflower)
Heliopsis helianthoides scabra (false sunflower)
Liatris spicata (spike gayfeather)
Perovskia atripliciflolia (russian sage)

please excuse any typos, my eyes arent the best for copying from books.