I'd do it the same way. You can directly assign a "Use Flow Rate Fraction Schedule" use to a WaterHeater:Mixed object without the need for a plant loop.

The I/O examples here actually list one that is pretty close to your needs, the one named "Stand-alone electric, tankless example", except you could change the "Ambient Temperature Indicator" to Zone and hook up your thermal zone to it, and it'll look like this:

WaterHeater:Mixed,                     ! Stand-alone electric, tankless example
      Tankless,  !- Name
      0.003785,  !- Tank Volume {m3}
      Hot Water Setpoint Temp Schedule,  !- Setpoint Temperature Schedule
      ,  !- Deadband Temperature Difference {deltaC}
      82.2222,  !- Maximum Temperature Limit {C}
      Modulate,  !- Heater Control Type {Cycle | Modulate}
      11712,  !- Heater Maximum Capacity {W}
      0,  !- Heater Minimum Capacity {W}
      ,  !- Heater Ignition Minimum Flow Rate {m3/s}
      ,  !- Heater Ignition Delay {s}
      ELECTRICITY,  !- Heater Fuel Type
      0.95,  !- Heater Thermal Efficiency
      ,  !- Part Load Factor Curve
      10,  !- Off-Cycle Parasitic Fuel Consumption Rate {W}
      ELECTRICITY,  !- Off-Cycle Parasitic Fuel Type
      0,  !- Off-Cycle Parasitic Heat Fraction To Tank
      30,  !- On-Cycle Parasitic Fuel Consumption Rate {W}
      ELECTRICITY,  !- On-Cycle Parasitic Fuel Type
      0,  !- On-Cycle Parasitic Heat Fraction To Tank
      Zone,  !- Ambient Temperature Indicator {Schedule | Zone | Outdoors}
      ,  !- Ambient Temperature Schedule
      <INSERT NAME OF THERMAL ZONE>,  !- Ambient Temperature Zone
      ,  !- Off-Cycle Loss Coefficient To Ambient Temperature {W/K}
      ,  !- Off-Cycle Loss Fraction To Zone
      ,  !- On-Cycle Loss Coefficient To Ambient Temperature {W/K}
      ,  !- On-Cycle Loss Fraction To Zone
      0.000379,  !- Peak Volumetric Use Flow Rate {m3/s}
      Hot Water Demand Schedule,  !- Use Flow Rate Fraction Schedule
      ;  !- Cold Water Supply Temperature Schedule

Of course, do change both the "Peak Volumetric Use Flow Rate" and "Use Flow Rate Fraction Schedule" to match your actual case.