I think people have an extreamly distorted view on how humid these things should be. I have a hard time believing that an enclosed system would ever need a humidifier running. Now if your system has outside air entering for venting is not almost totally closed for some other reason then this is a different situation.
Perhaps what you are experiencing is humidity drop as temperatures warm up during the day.. this is natural and completely fine as far as the plants are concerned. Regardless if you feel you still need a humidifier in the main terrarium to maintain humidity within that area. You will still need to rehumidify the A/C cooled air before it hits the terrarium. You think you have a humidity drop now during the day as the temperature warms up.. wait till you start pumping in a couple hundred cubic feet of cool air at 20% relative humidity.. The in tank humidifier will not be able to mix and adjust this incoming air fast enough IMO.
I suggest you talk to Swords who had a set up like this with an multiple in tank humidifiers and tried running the A/C air directly in without rehumidifying.