When I was reading your description of the site, Lavender was what came to mind. Also other herbs, like thyme and oregano/ marjoram. Think "mediterranean" for plants to grow there. Vinca seems to survive anywhere, but may be too invasive for you. Prostrate rosemary would give you ground cover too.

thanks beattie....

Some of the perennial geraniums are virtually evergreen and dont seem to mind poor conditions , especially the macrorrhizum group.

thanks, Inverglen