A small PNP transistor should work. Connect the transistor emitter to the battery, the base to your 5V supply through a resistor, and the collector to the LM3909. When the 5V is lost, the base resistor will go to ground, turning on the transistor and energizing the LM3909.
The base resistor should be sized to give adequate base current to saturate the transistor (0.1 times the collector current, or 0.2 times the collector current if you want to maximize efficiency). For one 1.5V battery, the resistor value would then be (1.5V - 0.65V) / (0.1 * collector current). The collector current is whatever the LM3909 draws when powering the LED.