I'm no expert, but I've heard that when it comes to Japanese Maples the common seedlings are much more sun and drought tolerant than the hybrids. I recently planted one in my backyard in a similar spot, sun part of the year and shade for the rest, and it's put on quite a bit of good looking new growth in it's spot getting a few hours of hot summer sun every day. They don't have the fancy leaves of the hybrids, but they do turn pretty colors in fall.

I have one in full shade under my Pepper tree. It's the common green leafed variety, but they get pretty big. Mine always has brown edges around the leaves by this time of year- I read that it is because of our alkaline water.

You could try Osmanthus Fragrans, or camellias.

Camellias are actually understory trees. As long as the shallow, sensitive root systems are shaded, they can handle inland heat quite well.

There's a California native dogwood, Cornus californica.

Some Manzanitas make lovely small understory trees. A horticulturalist remarked to me that they make perfect substitutes for Japanese Maples in California.

The native Toyon makes a nice understory tree. Slow growing. Birds love the berries.

Inland is hard on Japanese Maples. I'm coastal so they do pretty well here, but the first few years are tough.

I was going to also recommend a Cornus tree. If you want year round color, try tibouchina urvilleana (princess flower.) It blooms nearly all year long and does well for me inland in full sun and in shade.

There are many good choices. If you have not seen a really good specimen of 'Kerria japonica' in bloom, you should. They are easy to grow, need little water, no pest problems and when in bloom they are spectacular, with the brightest yellow bloom you have ever seen. Al

The Kerria is beautiful in the double Pleniflora form.

Douglas' maple (Acer glabrum) will do nicely in sun and should do OK in part shade, after your sycamores mature. They require less water than Japanese and vine maple.

Toyon should do well. Red elderberry (and Mexican elderberry, I believe) do OK in both sun and partial shade. The upright manzanitas tend to need either full or partial sun.