Foam pads are commonly used as insulation, they're just trickier to get in the right position. I've only ever used a pad and been as low as a bit under 30*F without any problems. Take a look at pad extenders that add foam wings to the edges of the pad to widen it. Because a hammock wraps around you, a wider pad is helpful.
The reason pads or UQs are important is because your sleeping bag does not provide much (if any) insulation when you lay on top of it because it is compressed. Leaf litter underneath would probably provide very little insulation. There is something called a Garlington insulator which uses leaves or other similar materials to insulate, but I don't know much about those.
Last edited by invisiblejules; 11-23-2012 at 16:02..