EnergyPlus can model 17 different types of air terminal units, but they are all preset for specific configurations (reheat vs. no reheat, constant volume vs. variable volume, etc.). None of these configurations include a humidification coil. Also, the Engineering Reference section for humidifiers doesn't mention any limitations on where they can be placed (central air system, outdoor air system, single-zone HVAC, etc.).
So, you're options are to:
1) Place a humidification coil at the outlet of the air terminal unit. The outlet node from the humidification coil is now the inlet/supply node for the zone. It sounds like you have already tried this.
2) Instead of placing the humidifier after the terminal, use a single-zone HVAC component in EnergyPlus that can contain a humidifier. It would then humidify "in parallel" to the VAV system supply air. I found one for containing a dehumidifier, but not a humidifier.
3) If those both don't work, place the humidifier in the central air system serving the data center. If the other occupied zones on the same floor as the data center don't need a humidifier, then you can create a separate VAV airloop for the data center on each floor.
