For the steady-state, uniform condition-- i.e. no changes in wind speed either over time or over space-- we can consider a glider flying within a moving airmass viewed from a reference frame fixed to the ground, as exactly equivalent to a glider flying within a stationary airmass (relative to the ground) viewed from a moving reference frame. There's no way to distinguish the physics at play between the two cases.
In other words, in an airmass that is uniform over time and space, if you can't actually see the ground, there is no way to tell which way the wind is blowing. There is no maneuver you can do where the aircraft will respond differently when flying upwind versus downwind versus crosswind.
If you are on the ground looking at a glider flying in a still airmass, it will be obvious to you that putting sails on the glider won't accomplish anything. If the glider continues to fly in the same airmass but you are now riding in a airplane passing by and viewing the glider from that perspective, would you think that putting sails on the glider would now accomplish something? Clearly not.
Expecting sails to work on a glider in the presence of a steady, uniform wind, is like expecting sails to work as you watch the glider from the moving airplane as it flies in still air.
Any theory that predicts that the forces acting on an aircraft are different when it is flying upwind versus downwind versus crosswind-- regardless of whether the aircraft has special "sails" or not-- is selecting the ground as a privileged reference frame. This violates the principle of Galilean invariance.
A sailboat is completely different than a sailplane (glider). A glider flies inside of the airmass. A steady, uniform motion of the airmass cannot exert any force on the glider. (This is true of vertical motions of the airmass, as well as horizontal motions of the airmass.) A sailboat exists on the boundary between two different fluids-- the water and the air-- so motion of one fluid relative to the other can and does exert a force on the sailboat.
There has been an attempt to "sail" an unpowered balloon in a direction different from the direction of the wind, but this attempt involved ropes dragging on the ground, so that the balloon could no longer be viewed as being "at rest" within the airmass. An equivalent would be a glider rising on a fixed length of cable that is tethered to the ground at one end, which is certainly possible on a windy day.
And then we have Dynamic soaring, which is a completely different "ball of wax", exploiting shear lines, wind gradients, the boundaries of thermal updrafts and downdrafts1, and other spatial or temporal variations in the airmass.
- Links relating to extracting energy from the boundaries between downdrafts and still air via "dynamic soaring" techniques: