A lot of the experts in here will have better ideas for you than me but you could use one of the ajugas, or any number of the low growing sedums? Maybe someone will know if the cotoneaster horizontalis (?) would like those conditions.

I know that "Aegopodium podagraria 'Variegatum'" spreads like crazy (approximately 36" within 2 years), is 6" tall, green and white with white flowers in the spring. Also called Bishop's Weed. I have seen people around here use it to cover bankings, etc and they manage to keep it in right where they want it.