As long as the pickup is submerged and fuel is going through the fuel pump, it is being cooled. The pump body does not need to be immersed in fuel for proper cooling of the pump. The only real problem here is actually running out of gas which sends air through the fuel pump (potential overheat of the pump motor) and potentially damages the fuel injectors because they are designed to inject fuel, not air.
When the gas light comes on in my Tacoma, it has approx 3.5 gallons left in the 21-gallon tank. In the 12+ years I've owned this truck, maybe 3-4 times I have driven it an additional 40-50 miles after the light came on, the needle was way below the E mark, still ran perfectly fine. Last time when I finally did fill the tank, it took 19.8 gallons which means there was approx 1.2 gallons left in the tank. Now 175,000+ ODO miles and no problems. I won't make a habit of doing this, but once in a while will not break anything. I have also noticed the lighter vehicle weight when driving with a near-empty tank... better handling and better performance all-around.