I think it's worth a try - I've used low tunnels inside my high-tunnel GH to protect plants early in the season. According to Eliot Coleman ("Four Season Gardening"), each additional layer of protection takes things up one "zone" (or is it 1/2 of a zone? can't remember for sure). And if you can add a little additional heat as per Daisy's suggestion, that certainly couldn't hurt either! Even just putting some jugs filled with water near the plants can help - they will warm up during the day and give off a little heat overnight, or at least moderate the temperature in their vicinity.
