It works okay in my location (maritime Pacific Northwest), but onces temps stay above 80 for a few days (something that rarely happens here), lettuce will start to bolt. No amount of shade can halt this process, unfortunately. Depending on you season length, you may be able to start another crop of lettuce in the early fall. Grown under a row cover or hoop house, lettuce does fine well into winter in all but the coldest climates.
I grow lettuce all year, by using other plants as a buffer from the sun. Your idea is a good one and also you can interplant along with tomatoes, peppers, etc. I would be careful about putting them with Sunflowers. Sunflowers have a aleopathic effect on some plants and may stunt their growth.
I use a raised garden bed ( Sq Ft. gardening method,)and actually have had good success (until August, the hottest month here in FL) , growing lettuce under our huge grandfather oak tree.It gets filtered sun coming in from the side but not enough to scorch it.
I’ve never grown lettuce successfully in mid summer; I don’t think shade would help with the temps here. I like the idea, though. I’ve used other plantings to provide shade for more delicate plants frequently. If your temps don’t get too high, it might work.
I’ve tried all kinds of things by now for my lettuces! Pole beans and sunflowers both… although those lettuces that didn’t get shaded still bolted. Last year I actually grew a separate lettuce garden next to the woods with a little fence around it to deter rabbits. I also sprinkled hot pepper sauce around and let my dog wander around the area. No rabbit damage and the lettuces were great. This year, due to the rain, I gave up and planted the lettuce seeds in with the regular garden, to be shaded by pole beans again. I also made a wooden frame kind of like a grape arch two years ago and let pole beans grow over that. That shaded the lettuces well but it also attracted my chickens and guineas, who liked to roost up there… and poop on the lettuce.