Hmmm, wood does indeed expand, but far far less than metal, and, true relative humidity would affect the wood but only to a small degree where the wood is covered in nitrocellulose, methinks.
I'm tending to the truss rod theory. My Warwick thumb which has a truss rod about 50% longer than a guitar does indeed develop a slight buzz in summer which disappears in autumn, so the neck curve must be changing.