Andrew Olson confirmed this. It was also confirmed by Frank Fuller as an early Alembic practice not necessarily for it's sonics, although it had to have had some attributes caused by it, but more for weight. Frank stated to me that they would also drill holes in the neck where the wings meet sometimes as well. Too much and the guitar won't be balance correctly. To answer your question, i do not know exactly how much of the wings was hollowed out, but it was.
"Tone is in the instruments. Technique in the hands. Do what you will." ~quote from some guy at the TGP forum
"Without a doubt, the quality of your instrument will effect your tone, and any pickups, effects, amps, and microphones used along the way will also have an impact on the sound waves."~Cameron Mizell