Hello, I always associate wall plants with aubretia trailing down.
Maybe you could consider Periwinkle too as its evergreen and has small flowers but not sure if it will trail enough for you.

Would a winter flowering clematis be too big?

Erigeron, pretty white and pink daisies which has been evergreen in my garden but maybe that was because it was in a sheltered spot. It grows quickly but can be trimmed easily and pulled out when it spreads.

How about, one of the evergreen thymes, Ivy-leaved Toadflax ( Cymbalaria muralis), Pennyroyal, any of the ground cover type Veronica's or low growing sage.

Seeing Honeysuckle's post reminds me of trailing rosemary. I grew one over a wall after seeing them grow in Spain and they are evergreen with pale blue flowers in the summer.