If it's in a greenhouse it should be fine. If you are still worried, cover the with clear or white plastic to trap the heat in and make sure the hydroponic system has enough water mass. Water has a greater capacity for heat than other materials, so while the temperature outside is below freezing, a large enough mass of water outside can be above freezing from the warmth during the day.

Fortunately lettuce is pretty cold tolerant so it should be fine. Hope it works out for you!