OK, firstly, I like this idea! Depending on the dog, its fur may be enough for basic warmth overnight, except for heat loss from the cold ground (which this addresses).
I might be missing something here, but why does the bottom fabric need to be breathable? Wouldn't it be best to just use the lightest, most "bullet-proof" nylon you can find? Cheaper, too!
Have you thought of using a non-outdoor fabric for the top? I'm actually thinking about "ultrasuede", a synthetic that feels like soft suede, is relatively light, and can be machine washed/dried. It's also really hardy, so sharp toenails wouldn't tear it.
If you use less durable, UL fabrics, the pad is less likely to last very long...