We study the impact of employing Unmanned Aerial Vehicles (UAVs) flying random, arbitrary missions as purely-opportunistic relays for cooperative awareness applications in vehicular networks. We do not require that opportunistically relaying UAVs alter trajectory nor speed, so that the additional relaying task can be executed with close-to-zero impact on the execution of the primary mission. Based on extensive computer simulations we demonstrate that, within a wide band of acceptable speeds, flight routes (up to a standard deviation of 300 m from the optimum), as well as altitudes, opportunistic relaying of transmissions via UAVs can yield a benefit to system performance that is on the same order of magnitude as that of optimally deployed UAVs. Moreover, much of the reduction in impact due to suboptimal missions can be recovered simply by moderately increasing the number of UAVs.