We study the suitability of Unmanned Aerial Vehicles (UAVs) as purely opportunistic airborne virtual network infrastructure to support urban wireless networks, specifically in two Vehicle-to-Everything (V2X) use cases: First, UAVs being used as relays for cooperative awareness applications; second, UAVs being used to coordinate channel access for platooning in urban areas. We do not require that these UAVs alter trajectory nor speed from those of their random, unrelated primary missions, so that these additional tasks can be executed with close-to-zero impact on the execution of their primary missions. Based on extensive computer simulations we show 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. We further show that an opportunistic channel access control can reduce the total number of packet collisions by approx. 86 % compared to a scenario without any UAVs. Moreover, much of the reduction in impact due to suboptimal missions can be recovered simply by moderately increasing the number of UAVs.