You know what I ALWAYS hear that Californians grow in some windowbox or pot in those conditions? Nasturtiums! There are some that are gorgeous, and they will fill up the container like a little bush and trail down the sides. And they bloom all summer. You could also try some of the low-growing lantana or even some million bells or mini-petunias (which I noticed alot of places around here are selling instead of the million bells). You might even try a mini bougainvillea since they seem to mainly bloom when stressed - ie., when they are hot, dry, and potbound. lol

To try to reduce the watering, consider gettng some moisture-retaining crystals like SoilMoist into your soil mix and that will help.

What part of Northern CA are you in?
That would be the most important thing to know.
What people are growing in full sun in SF wouldn't last out in Walnut Creek's 100 degree days.
Some plants for very hot sun are:
Vinca (I have them planted in very shallow box).
They are both on the short side.
Marigolds (an old standby, but they do take the heat).
Million bells (Calabracha) These will cascade.
Lantana is good, however I've only seen it sold in gallon containers.
I always take a look around and see what the City is planting in full sun on the streets and in the shopping centers.
I would also consider mulching to keep the tops moist.

Sorry to be boring..but I find that geraniums flower all of the year except for two months. Nice to be guaranteed some flowers.

I think lavender looks really smart in window boxes but I don't know which type to get to be compact enough :-( My lavender in a pot goes all over the place.