"So clearly, anything that flowers won't work. "

I'm not sure why you would say that. Any deciduous shrub is fine in winter shade (no leaves so no need for sun for good photosynthesis) but should bloom well in summer sun if that is its preference. And a fair number of BLE's should work as well.

I'd visit a local independent garden center or nursery (not box store) and see what they have that fits your requirements. You should have quite a bit to choose from.

I have some Sweet Lavender bushes that get zero sun in winter, and partial sun in summer, and they're happy. Some others (same type) in a different spot get full sun in summer and partial sun in winter and they're fine too. They are about 3' tall and 5' in diameter, not including the 2' flower stalks, and are a nice green green (not grey green like most lavender).

Or maybe, Sweet Pea Shrub? (Petite Butterflies seems to be about the right size you're looking for). They grow quite well in a variety of situations in my neighborhood (Westchester, so a little more coastal than you).

Some Ceanothus varieties might work in that spot too, if you find the types that can tolerate the shade. I have a Ray Hartman growing happily in almost full shade, but that's too large for your spot. I seem to remember there are quite a few that can handle shade - there's a chart out there on the internet somewhere that listed them by light requirement.

Have you considered going in a different direction? Like possibly a fence or even a canvas-backed trellis?

By "died after the winter" do you mean perennials that died after this recent, very wet winter? If so, that suggests a drainage issue. Penstemon is picky, but the others are pretty sturdy. A picture of the site might help. What plants do you see thriving in your neighborhood? They might be good choices. We can help you identify them.

Rosemary, Westringia, breath of heaven, Knockout roses, Kaleidoscope Abelia, bigger Abelia, Raphiolepsis, Pittosporum, fortnight lilies, society garlic, and star jasmine are some other sturdy landscape plants that are commonly planted around San Diego.

Good suggestion from gardengal to check with an independent nursery. We had to do that yesterday, and the help we received was much appreciated.

gardengal, we went to the nursery regarding the question I posted here about watering after the rains and brown leaves on Pittosporum. We took some leaves with us, and they told us it was the dieback you mentioned. Just wanted to say thanks! :-)