Let me preface this by saying I'm aware this is not the intended use for this object. However, I believe, this is the only option for an infinite capacity plant loop object that's not included in the default EUI reporting.

Loop Setup

• There can't be a bypass pipe on the supply side.

Rudimentary EMS Code

EnergyManagementSystem:Sensor,
  CW_Loop_Outlet_Setpoint,              !- Name
  CW Loop Outlet,                       !- Output:Variable Index Key Name
  System Node Setpoint Temperature;     !- Output:Variable Name

EnergyManagementSystem:Actuator,
  Temperature_Source_Setpoint,          !- Name
  HP-Loop-Temp-Schedule,                !- Unique Component Name
  Schedule:Year,                        !- Actuated Component Type
  Schedule Value;                       !- Actuated Component Control Type

EnergyManagementSystem:Program,
  TemperatureSourceControl,             !- Name
  SET Temperature_Source_Setpoint = CW_Loop_Outlet_Setpoint;

EnergyManagementSystem:ProgramCallingManager,
  TemperatureSourceControlCall,         !- Name
  AfterPredictorAfterHVACManagers,      !- EnergyPlus Model Calling Point
  TemperatureSourceControl;             !- Program Name 1

Output

• Outlet Temperature = Outlet Setpoint WHEN Outlet Mass Flow > 0
• Outlet Temperature = Inlet Temperature WHEN Outlet Mass Flow = 0