Lighter heat sealable?

Does that fabric remain airtight under pressure ?

Most likely you could heat seal fabrics that have coatings too thin to be airtight – or at least to stay airtight for more than a very short useable lifetime. Functionally waterproof coatings are not necessarily airtight, and the very light fabrics are unlikely to be airtight since their coatings are just enough to be waterproof.

In trying to answer the airtight question myself I applied a little common sense. Hydrostatic head measures the pressure of the height of a water column in millimeters. This can be converted to PSI. Even if air interacts with the coating differently than water, it's probably a good proxy. A HH of 1000mm equals 1.42 PSI, so that PU4000 should be good for 5.7 PSI. Any one know what kind of air pressure sleeping pads commonly see? 200 lbs over a 6" square produces 5.55 PSI. That suggests PU4000 will just work, but I suspect the physics may not be that straight forward. Going from 3osy to 1.4osy should be good for dropping about 4oz on a 72"x20" mattress. Sure would be nice if it works :)

I don't think using a fabric not designed for that purpose will work. Firstly, the PU coating on waterproof fabrics is a different material that the heat sealable "thermoplastic polyurethane", TPU. It won't seal the same way, some glue would be needed. Secondly, as it was already mentioned, waterproof fabrics are most likely not airproof, or they will develop some leaks very quickly.