In cross-border mobility scenarios, interactions between mobile network operator (MNO) infrastructures and multi-access edge computing (MEC) systems across international borders are required at both the control and data planes to provide seamless and uninterrupted services to users transitioning between two MNOs. However, the exchange of information between MEC systems faces numerous challenges, as it necessitates the exposure of data regarding resources, network information, and application instances. This paper proposes a MEC federation architecture based on the use of an open platform and an Open Gateway network-as-a-service (NaaS) model to facilitate cross-MNO MEC service orchestration and inter-MEC communication for connected and automated mobility (CAM) services when crossing international borders. The proposed architecture is validated using an infotainment use case in a cross-border scenario with two 5G standalone (SA) networks. The evaluation results show significant improvements in service continuity, with short interruption times and high reliability, highlighting the practical benefits of MEC federation in real-world applications.