Hi all -
Just wanted to share a tip that works for me, and ask the more knowledgeable folks on the forum whether they can see anything that might ultimately be harmful with this approach.
To keep my guitars at the right humidity while in the case, I use Planet Wave Humidipaks, which are basically vapor permeable salt packs with an equilibrium vapor pressure equivalent to ~45-50% humidity. You slide them into a little cloth sleeve, and two packs sit in the soundhole and one in the headstock area of the case. The idea is that they then either release (if the humidity is low) or absorb (if its high) water vapor to keep a relative humidity of ~45-50%.
In the winter, my experience is that these packs dry out and get crunchy pretty quick (in about six weeks or so, given that I'm opening the cases on a daily basis). They aren't rock hard yet, but they feel pretty dry. In order to keep from having to buy refill paks, I've taken to periodically putting the packs in a large Gladware-type container along with a small bowl of hot water, and in about 4-6 hours, they soften up quite nicely and are ready to go back into the case.
Other than Planet Waves being out a couple of bucks, I can't see anything wrong with this approach - but I'd welcome thoughts from chemistry types who know more about desiccants and physical chemistry than I care to try to remember from college.