Any zone 8 or above have these survive with some foliage and above-ground stems through the winter?
They do ok in OS but we're z8, but a cooler z8 than you are in San A. they do NOT do well with immersion in salt water, eg Katrina flooding ;-)
I got a Fatsia japonica 'Spiderweb' through the winter in the Washington DC area, with some protection (wrapped with burlap & stuffed with dry leaves). All the leaves were killed, & much of the stem suffered die back. It is already putting out nice new growth, so hopefully it should be in good shape.
I have been growing Fatsia japonica in my garden (zone 7a/b) for 4 years. I grew them (from seed) indoors for a year, put them out in the spring and covered them the first winter in the ground but since then have only mulched them heavily with fallen leaves. They have weathered numerous single-digit temperatures with very little foliar damage.
It's quite a shock to see the fatsias after a particularly cold night. The leaves turn blackish-green and go limp, looking a bit like boiled spinach. Their recovery when it warms up again is almost miraculous. The worst damage seems to come in early spring when some of my plants start to get direct sun because the trees haven't leafed out enough to give them shade.
I just got some yesterday and planted a 5 gallon plant near a fence, between two houses in mostly shade. I expect it will do fairly well if we don't get to or below zero this coming winter.