Imagine plaster or bluestone with corten for the outdoor dining room retaining wall. This is an expensive installation but I like it. Maybe we could use only two vertical insets along plaster dining room wall. Picture the corten base as a cap for the wall. I like the idea of a mini succulent garden inset into the corten cap. (Kind of similar to your small rock /plant idea set into the patio.) It would be more at eye level and a focal point where the expansive yard landscape is juxtaposed against smaller / intimate plantings.