This is probably too late to help you, but just in case anyone else wants to know... I would plant the seeds in oasis or rockwool cubes. Wait until the roots hit the bottom of the material before you transplant them into NFT or float culture. If you're planning on growing them in media culture, you can plant them directly. You can cover the seeds with a very thin layer of vermiculite (this is done to help hold more moisture against the seed) but be careful because lettuce seeds want some light to get through.