Simple represenation of HVAC to match building metering

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Not sure I completely follow the question.

If you want to use a dummy zone, you can easily use ZoneMixing to link it to other zones. But note that ZoneMixing has no impact on the source zone, only on the receiving zone.

A better option may be to use EMS (EnergyManagementSystem) to control the ideal loads system. If you have flow and supply temperature data, then you can use EMS to specify the ideal loads mass flow rate, temperature, and humidity at every timestep. But note there is a known issue regarding this. If the ideal loads object doesn't think there is a zone load, then it will be off, no matter what the EMS flow rate says. See issue # 5788 for suggested workaround.

Sounds like you have a DOAS serving zone HVAC terminal units. It seems like you could use OpenStudio to model this (would help remove some of the complexity related to airflow branches, etc.) and have an air loop representing the DOAS (what you call the AHU) and assign ZoneHVAC to the thermal zones. You can use something other than IdealLoads and just let it autosize. That way you can have constant flow systems.

In terms of metering the AHU and zone equipment separately, there may be a way of doing this, but I'm not familiar with that at the moment. As a last resort, you could use the output variables for fan and heating and cooling coil energy and DView, or I personally like the free DesignBuilder Results Viewer, to sum up the energy use independently.