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Green Infrastructure promotes the use of natural functions and processes as potential solutions to reduce negative effects derived from anthropocentric interventions such as urbanization. In cities of Latin America, for example, the need for more nature-based infrastructure is evident due to its degree of urbanization and degradation of ecosystems, as well as the alteration of the local water cycle. In this study, an experimental approach for the implementation of a prototype is presented. The prototype consists of a gray-hybrid element for first flush bio-treatment and runoff detention, adapted to the existing stormwater sewer. The experiment took place in a highly urbanized watershed located in the Metropolitan Area of Costa Rica. The characteristics of the existing infrastructure in the study area at different scales were mapped and compared using the Urban Water System Transition Framework. Subsequently, preferences related to spatial locations and technologies were identified from different local decision-makers. Those insights were adopted to identify a potential area for the implementation of the prototype. The experiment consisted of the adaptation of the local sewer to act as a temporal reservoir to reduce the effects derived from rapid generation of stormwater runoff. Unexpected events, not considered initially in the design, are reported in this study as a means to identify the necessary adaptations of the methodology. Our study shows from an experimental learning-experience that the relation between different actors advocating for such technologies influences the implementation and operation of non-conventional technologies. Furthermore, the willingness of residents to modify their urban environments was found to be associated to their own perceptions about security and vandalism occurring in green spaces. The implementation of the prototype showed that both the hydraulic performance is relevant for considering it as a success, as well as the dynamics of the adapted element with the existing urban conditions. In consequence, those aspects should be carefully considered as the design factors of engineering elements when they are related to complex socio-ecological urban systems.