Well, I didn't know about this surface sowing of lettuce but I had a great germination rate with different kinds (also fristina, thanks Tim). My first sowing is ready to be planted in the greenhouse and I have just sown the second batch for planting outside later when the peppers and tomatoes will fill the greenhouse.
However, the icebergs are not doing well at all. Sown at the same time as the first batch of leaf lettuces, only 3 of the 12 have come up, they had 1 long leaf showing for a long time and just now I can see a grow point where others seem to be developing. I don't remember them doing like that in the past.
Btw, I always put a row lettuces down the center of the bean pole tents. When they are just about mature, the beans will shade them and they seem to like that because the ones in the open will start shooting much sooner after it gets really warm.