djpsr,
An interesting project that I have thought about myself.
For my display case type humidor (72"'H X 42"W X 15"D) I use the digitally controlled humidifier from Habitat Monitor. I believe Bob Staebell uses Habitat Monitor controls in his Aristocrat humidors.
http://www.habitatmonitor.com/
They have units that can accomodate room sized humidors, so you might want to check them out.
The advantage of an active humidification system is that you can turn almost any type of cabinet (or closet) into a humidor.
... rjs