From what I've read you can plant your fall crops starting in mid September, with broccoli seedlings going in starting late august.

I started my seeds too early this year too, somehow. Last weekend, I potted them up into bigger pots and put them back in the basement under florescent lights hoping to keep them out of the heat awhile longer. I did plant some broccoli plants out in an area where they get afternoon shade. They look really good. Just hope they don't bolt in the heat.

I always failed with Brussels Sprouts.
This time I planted them in July, as recommended by some websites. Still small though.

Below is another website for reference of planting...

Here is a link that might be useful: Southern California Planting Schedule

Unfortunately that link is only good for sunset zones 23 and 24, which is along the coast. It doesnt help out people that are more inland.
Here is a better planting schedule

Here is a link that might be useful: California Planting Calender

Yes, some amount of shade can help, as can a heavy layer of mulch to help shade and insulate the ground. We're still in the high 80s-low 90s here, though nights are cooling off, and my transplants are in the new high tunnel under a layer of floating row cover. That seems to take the edge off the heat enough that they don't experience wilting from heat stress every day. Don't know if your outside is hotter than inside the tunnel, though. :)

I agree with sunnibel. I plant brassicas as early as late August and I'm about 20 miles inland from San Diego... where our summer doesn't really start until about then. high 90's and 100's are not out of the norm for the 1st couple months of growth, but shade and mulch(especially) help regulate the soil temps. Once they get a 2-3 week hold in the soil, shade is removed completely.

Snow peas I start in late October/Nov.

Kevin