I don't think I have seen lettuce very much during the hottest part of the summer where we are in Greece. I suppose you could try the more heat tolerant varieties, like Jericho romaine lettuce, which is said to be bred in Israel. Some kind of shade to protect them during the hottest part of the day would probably be useful, but the problem would be to keep the soil cool enough during the germination period, lettuce seeds don't like temperatures above 23-24 degrees C.