Looking for a lighter and more waterproof pack cloth, the thought of balloon cloth came to mind. It must resist the elements, be strong enough to resist fatal punctures, have close to zero porosity, and yet be light enough to float through the air. But to find out how waterproof it is, it would be necessary to find some.
Right away, web searches suggested that quality balloon makers control the manufacture of their own cloth, and order in large quantities at great expense and for their exclusive use. A quality cloth is called "Hyperlast," is discussed on the site for Cameron Balloons.
The material has a base weight around 2.3 oz/sq/yd, is silcoated on both sides, treated to resist UV, made of tightly woven Dupont 6,6 nylon fiber, and claimed to be eight times stronger than "conventional ripstop nylon."
Another balloon cloth, slightly heavier, and PU rather than silcoated is available from from Extremtextil at http://www.extremtextil.de/catalog/Nylon-PU-coated-balloon-fabric-100g-sqm::1802.html
(Note: ExtremTextil does ship to North America, but now that international money orders are no longer commonly available, I've had trouble ordering from them; although I'm able to order from companies in France and the UK with a PayPal account, so it is possible this problem can be remedied.)
However, I kept looking for the silcoated variety, and came across an eBay page that sells scraps and swatches of balloon cloth with specs similar to Hyperlast on its eBay page: http://www.ebay.com/itm/320967786124?ssPageName=STRK:MESELX:IT&_trksid=p3984.m1555.l2649
The seller may be contacted through 'Ask Seller Questions' on the page.
After some correspondence, I purchased several large swatches of balloon cloth, enough for several packs. Some in royal blue weighed in at 2.75 oz/sq/yd, and in a gray ripstop with silver backing, about 2 oz/sq/yd. The silcoat can be regularly refreshed with Camp Dry silicone spray. Both fabrics are densely woven but supple, very water resistant, and will be used for pack making as soon as possible. I find this nylon material much more suitable for sewing than the heavier Cuben and Xpac laminates I've tried, and am willing to bet that it will hold up a lot better.