Well, until the middle of July at least.
What's the secret? First, the lettuce bed gets morning sun and afternoon shade, which helps slow the bolting. All-day filtered shade would work even better. Secondly, I chose varieties that claim heat tolerance - these are Rouge Grenobloise and New Red Fire, but others work as well: read the catalog descriptions and select wisely. Judicious regular harvests would probably help as well, though what these plants got was an occasional razing when I arrived and said, "What? Still lettuce? This won't last another week!" and cut it all to the ground.
My experiment at home, growing June-planted lettuce in between tomato plants in raised beds, worked pretty well too, although I've had to discontinue harvests because the lettuce is between tomato plants and therefore I can't reach it. Better planning next year will help with this issue.
If anyone manages to get their lettuce edibly through 100+ temperatures this weekend, I'd love to hear about it.