My gamble payed off. Using small 1x2mm circular rare-earth magnets I been able to secure sails to my ancient galleys. Using superglue (of which mine isn't!) I was able (eventually!) to secure the magnets to both the galley and sail.
This now permits me to affix the sails when I want to but leave them off most of the time, as would occur prior to most battles. Below you can see the arrangement of the magnets on both the galley and sail. A 1x2mm round magnet is powerful enough to secure a large Xyston miniatures sail, though it is a bit flimsy. The smaller sail secures more firmly and will be the main type of sail I apply this method too. I'm also only using one sail on a galley.
Below are a few close-ups showing the magnets affixed and the removable larger sail.