Instead of having the sail on slides running in a slot on the mast, put
a vertically tensioned cable just aft of the mast, say 2 cm back
running from boom to top of mast. The sliders will be rings around
this cable and will be unable to bind.

Being seriously radical, get rid of the vertical mast and replace it
with a sort of vertical wishbone with a vertical cable running its
length down the middle where the mast would be. You would have
conventional fore and back stays but the spreader and shrouds would be
different. The sail RINGS would run up and down the vertical cable.
Furhtermore, this sail would be more efficient than one with a
conventional mast as the airflow would not be disturbed by the mast.