It's always been my understanding that drying and cracking of fretboards is best prevented by a few drops of lemon oil rubbed well in, once a year or so. There seem to be newfangled brand-name fretboard treatments out there now, but I imagine it's the same idea (somebody please correct me if I'm wrong). I wouldn't expect an in-hole humidifier to have any effect on the neck or fretboard.
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