Interoperable distributed energy resources (DER), including photovoltaic (PV) inverters, are capable of providing a number of grid services by receiving commands from grid operators, aggregators, or other third parties. In many control scenarios, the grid operator must determine the operating mode and parameters for the devices to achieve a specific control objective. In this paper, we experimentally validate a distributed technique to achieve optimal DER reactive power operating points for distribution circuit voltage regulation using extremum seeking control. The method is demonstrated with physical and virtual DER connected to multiple hardware-in-the-loop distribution circuit simulations. This paper demonstrates PV inverters with realistic communication interfaces can receive real-time control signals from an aggregator and adjusting their reactive power to minimize voltage deviations. Simulations were conducted with fixed and variable solar irradiance to demonstrate the robustness of the approach.