While daydreaming some time ago I thought of a possible technique for making sails. One of the most archival fibers around is linen, but it is difficult to find linen cloth in a fine tight weave. A company called Light Impressions makes an acid free finely woven linen tape with a water soluble adhesive backing. The link is: http://www.lightimpressionsdirect.com/acid-free-linen-tape-white-1-x-20yds/archival-adhesive-tapes/ .
I think it might be possible to cut the tape into strips to match the panels on the real sail and then glue the panels together by the edges using the adhesive backing. The panels could then be stitched together and the bolt ropes and other details added. Finally the sail would be soaked in water to remove the adhesive backing, the panels now being held together by the stitching.
I am not planning on building any models with sails in the near future but perhaps someone who is could try this technique and let all of us know if it works.