Irish moss would be nice...Sagina subulata. RJ

Here is a link that might be useful: Irish moss

Ferns especially maidenhair speenwort, ebony speenwort, Virginia polypody, fragile fern, berry bulblet fern and Japanese painted fern are some possibilities.
Flowering plants; Ramonda, Campanula rotundifolia, other low-growing campanulas, Corydalis lutea (blooms from spring to fall). A lot of plants have self-seeded into my walls. if you have seeds of rock garden plants or other small plants, you might try sowing seeds in the crevices.

violets are nice too, and easy to grow in small spaces between rocks, but very prolific!

They're herbaceous, but some creeping Sedums would add interest. I know 'dragon's blood' can take mostly shade. Slightly less colorful that way, but still an eye-catching plant.

what about ajuga? in mild winters it is evergreen, flowers in the spring. quick spreader.

another vote here for Corydalis- I had one drop seeds without knowing what would happen- now, I have five little ones in cracks in my cement wall, plus about 15 others which I have moved to other areas of the garden- the flowers aren't much, but the lacey foliage is to die for :)

i have a creeping sedum that volunteered in my yard, which i transplanted into my bed only to have it take over there... wonderful bright green plump creeping foliage and tiny little yellow blooms in late spring/early summer. i threw some down below my retaining wall where there is limited sun and lots of rocks, and this year, well! there are a few more where i DIDN'T put them. and i've been thinking about what to put there at minimal cost to cover all the rocky slopes i have.

try sedum, everyone says they are for sun, but my experience has seen different. better yet, ask me to send you some in an exchange. :P