Future cooperative autonomous vehicles will be able to organize into flexible platoons to improve both the efficiency and the safety of driving. However, platooning requires dependable coordination through the periodic wireless exchange of control messages. Therefore, challenging propagation scenarios as found, e.g., in dense urban areas, may hinder coordination and lead to undesirable vehicle behavior. While reconfigurable intelligent surfaces (RISs) have been advocated as a solution to improper coverage issues, no system-level simulation exists that accounts for realistic road mobility and communication aspects. To this end, we present one such simulator built on top of the OMNeT++-based PLEXE and Veins frameworks. Specifically, our contribution is a simulator that takes into account vehicle mobility, physical layer propagation, RIS coding, and networking protocols. To test our simulator, we implement an RIS-assisted autonomous platoon merging maneuver taking place at an intersection where the absence of any RIS would limit successful communications to an area dangerously close to the intersection itself. Our results validate the simulator as a feasible tool for system-level RIS-assisted cooperative autonomous vehicle maneuvering, and ultimately show the benefit of RIS as roadside infrastructure for wireless coverage extension. © 2023 IFIP.