Bioeconomic models make it possible to assess ex-ante the consequences of public interventions in the agriculture sector. Yet since policymakers rarely participate in their design or use, these models are often too complex or do not address their questions. This article describes the implementation of a nine-step participatory modelling process involving regional policymakers, and analyses the contribution of this process to the cross-brainstorming between policymakers and researchers. In the preliminary stage, researchers develop a prototype bioeconomic model which links the farm level, where trade-offs between productions are considered, with the regional level, where the balance between production and consumption per product is considered. Public actors are then mobilised to identify issues and characterise the scenarios that can be used to explore the questions selected. The scenarios are simulated with the bioeconomic model, which evolves depending on the needs, and the outputs are discussed with the public actors. The method was applied in La Réunion (France) on the design and assessment of aid mechanisms related to the development of organic farming. The results showed (i) which farms convert to organic farming according to aid amounts, (ii) how local demand per product is satisfied and (iii) what are the amounts of the ensuing budgets for subsidies. The approach proved to be relevant for exchanging knowledge between public actors and researchers, regarding in particular innovative farming systems, farm diversity and multi-criteria evaluation of policy choices. The simulated scenarios offered powerful support for discussions about market evolutions and environmental challenges. These results indicate that transparent modelling tools and long-term partnerships with policymakers are required to implement such an approach at the regional scale.