It would not work unless you had shorter lines (10 meter say), even then there gets holes in the wind from the fans, eg in the center of the fan and on the outside edge, this creates a gusty effect, plus one side of the pool blows 20kn and on the other its more like 16kn. You would have to have the ceiling approx 100m high with fans floor to ceiling, be 100m+ in width to allow for downwind landings = not worth it.
A better idea might be making a huge man made lake in a really good consistantly windy spot

PS - i remember the first time they had a comp in the indoor windsurfing place, they put blueloo in the pool to make the water look blue. Everyone complained. lmao