Paradigm shifts in electricity generation are leading to more renewable and distributed energy resources (DER) on the grid. There is a strong interest to utilize these resources for various grid services, but the practicality of commanding multiple DER is often an obstacle to such approaches. In this article, we seek to validate through Hardware-in-the-Loop (HIL) simulation an extremum seeking (ES) based control scheme proposed in previous work. The method aggregates and coordinates multiple distributed controllers to offer transmission grid level services. The HIL validation uses a prototype real-time implementation of the controller's logic on distributed devices and photovoltaic (PV) inverters operating on a simulated utility distribution feeder. Several grid services, including load following and voltage regulation, were validated to demonstrate the deployment feasibility of the ES control approach on equipment already installed on the grid.