Actually, you and I seem to have hit upon the same notion (you get the credit, though, because you published first). I have been considering almost exactly what you described, and the heat-sealable nylon that I ordered from Seattle Fabrics to put together a prototype just came in the mail.
Several potential complications occurred to me, though. First, it might be better for the mat to be pretty thin (less than 1.5"), to keep the load as close to the back as possible. Also, a lot of my hiking is done in the heat of summer, so I don't really want the mat to be a very good insulator when it is against my back. In fact, I considered incorporating open air channels in mine. This would result in a section of mat that was thin and not warm, though. A thin sheet of EVA foam could be carried to augment the warmth of that section of mat, but this negates some of the weight savings of this idea.
Not a bad idea, just some problematic details.