It's been interesting watching this thread blossom over time. The action you're aiming for is very low, and it seems to me that you may be suffering from the unfortunate confluence of a neck which is more sensitive to environmental conditions as well.
If you have seen John Entwistle's book Bass Culture which documents many of the instruments in his collection along with notes, you'll see that he (also famous for playing with low action) had many of his Alembics and Buzzards built with graphite necks (by Modulus) so he would have any chance of maintaining a playable instrument on the road. I remember visiting Modulus around the time they had received one of his prototype Warwick Buzzards to be reproduced by Modulus. They were just getting ready to carve the neck buck (master for the graphite mold) carved to duplicate the wood neck profile and contacted his road tech to make sure it was set up properly. They were shocked to find that it had been sent exactly as John played it, which was a virtually unplayable setup for anybody else. You may have seen pictures of the autographed green Modulus Buzzard which was ultimately raffled off in Bass Player magazine. This was one of the first Modulus Buzzards completed which was delivered to John as The Who tour passed through Oakland and was rejected because - if you can believe this - the action was TOO LOW (it had some non-functional neck LEDs too). I played it when it came back to the shop and it really almost impossible for me to tell when I was fretting a string or not (I play a pretty high action, so it was totally buzz city for me). They asked if I was interested in buying it, but it was pretty eye-blinkingly expensive (well, maybe not to Alembic folks), even at the buddy-buddy price which basically reflected the labor and material costs. And who (should that be Who?) dares actually play a Buzzard in front of people?
Wood is a natural product and even with choice selection and additional strength from laminations, you won't know how it turns out until it's done. This neck might be fine in the hands of somebody who plays with higher action so there's not really a defect other than the issue of suitableness for the owner.
If this is indeed an issue of humidity (I think it's that much more than these small variations in barometric pressure), it seems to me that applying a heavy coat of wax to the fingerboard may help. The back side of the neck is largely isolated from the environment by the varnish, but the fingerboard is totally exposed. As the humidity (or pressure) goes up and down, it's much more likely that the fingerboard is flexing the neck, a coat of wax may reduce that effect. I'd recommend finding a pure carnauba wax for this, as it's harder and has better sealing capabilities. You should probably avoid waxes with silicone in them (this would be any of the easy-on auto waxes) as it makes the fingerboard blotchy looking and hard to refinish. You can get pure carnauba wax at a woodworker's supply, or auto paint store (and yes, it will be that one can of wax that's about 3x the cost of the others).
Wax is easy to do. If it helps, then you might consider having the fingerboard finished with polymerizing penetrating oil, like tung oil. This will have the same effect but is much more durable.
Another thing to check it so see whether you need a fret leveling. With tolerances so tight because of low action, you're much more susceptible to buzzing because of a high or low fret as well as localized warping. If you have a good fret leveling performed (perhaps even have some extra relief cut into the fret tops in the middle) you may find that the climatic variations don't bug you as much. You might try seeing whether the fret tops are really level with a straightedge when the playability is problematic. All it takes is one high fret in the upper octaves to make for a very buzzy instrument.
Good luck,
David Fung
