I've seen recommendations to plant the seedlings in February, which
is insane in Sydney -- they keel over in the hot weather. Problem
is that planting them later doesn't give them enough time to grow.

Shade cloth?

But yeah, getting them started in the hot weather is usual problem
with fall crops (even more so here in Washington, DC, where most fall
crops need to produce by winter, as we get a few months of freezing
weather, and only a few fall-planted annuals will overwinter).

http://chookiesbackyard.blogspot.com/

Nice blog. As for growing plants with limited access to water, have
you ever seen or heard of the underground gardens of Fresno, CA?
There are some photos at http://www.forestiere-historicalcenter.com/
but the basic idea is that plants like grapes or citrus are planted in
underground rooms, with their leaves growing up through open
skylights. Not sure how I'd apply this to any other situation, but it
is always what I think of when I ponder gardens in dry climates.
There's probably a garden technique lurking here (cylinders of shade
cloth or opaque cloth open at the top? I don't know).