Public displays play an important role in information dissemination -- market reports highlight the increasing number of displays deployed. Due to the often prominent placement of public displays in the physical environment, displays can play an important role in the dissemination of trusted content, particularly during emergency situations. In order to leverage displays in emergency situations however, appropriate content creation and dissemination technology is key to allow display and space owners to efficiently distribute important information and target affected user groups. In this paper, we present our lessons learned from designing and developing an emergency messaging system in the context of a large public display testbed. We provide insights into two design probes and feedback captured through focus groups with stakeholders of the display network. Based on the feedback, we provide insights into requirements captured and provide a discussion on lessons and design considerations.