This paper reports a cross-case analysis of three summer writing programs for youth in the northeast United States, each a longitudinal study in the tradition of design research. Initially, all three programs were most concerned with leveraging cultural and linguistic diversity as a resource for multilingual participants. As the three principal investigators played the role of critical friend informally for one another, they identified a common commitment to inclusive writing pedagogy-instruction designed to promote writing proficiency and engagement for all students, including but not limited to students with disabilities. Employing lenses from disability studies applied to literacy, they undertook joint analysis with this focus. Common data sources across the sites included documents describing the programs to varied audiences, instructional artifacts, communication among team members in each site, students' print-based and digital writing, and pre- and post-program surveys. Cross-case analysis with a design research heuristic adapted from Bakker yielded two cross-cutting design principles. To construct inclusive writing pedagogy in the context of summer programming, educators are advised to: (1) Build writing communities deliberately, by promoting a sense of belonging, and (2) Promote a broad vision of who can be a writer, by offering varied writing models and exemplar texts. The study concludes that research-based, cognitively-oriented instructional approaches are a necessary but not sufficient condition for supporting writing competence for youth writers with varied needs, including those with disability labels. Emphases on relational and social aspects of writing are essential as well.