Engaging with non-existent and non-identifiable future generations poses ontological and methodological challenges for sustainability discourse. This paper presents a workshop methodology that deploys AI as a configured persona-mediator alongside participant co-creation of 2050 personas and structured multi-phase dialogue. The methodology addresses how AI’s analytic role in participatory futures should be specified. The methodology operationalizes Future Accountability, a trans-generational framework distinguishing four interrelated dimensions: social empathy, co-existence, shared responsibility, and shared goals. We report findings from a workshop with 12 Korean participants drafting a declaration on a hypothetical UN Plastic Treaty. Deductive thematic analysis of 287 utterances with dual-rater reliability (Cohen’s κ=0.97) revealed that each dimension operates not as a unified capacities but as plural registers: social empathy emerged across three registers (possessive, ontological, and cross-cohort), and co-existence operated across two (constitutive, in foundation phases; reflexive in final reflection), jointly accounting for 83 % of persona-construction utterances. Two groups receiving identical AI configurations produced dimensional trajectories that diverged by over 50 %age during declaration drafting, indicating that AI design configures a possibility space whose outcomes are shaped by participants situated practice. The study contributes (1) a workshop methodology integrating AI persona-mediation and co-creation personas as a concrete mechanism for trans-generational engagement; (2) empirical evidence of plural-register operation in social empathy and co-existence, refining the framework’s analytic granularity beyond unified capacity formulations; and (3) a design-study formulation that reframes AI persona design from a technical solution to a configured interaction specification mediated by dialogue design and situated participant practice.