Could, e.g., a custom-made 100ktoken summary of the architecture and relevant parts of the giant repo and base index of where to find more info be sufficient to allow Opus to take a large task and split it into small enough subprojects that are farmed out to Sonnet instances with sufficient context?

This seems quite doable with even a small amount of tooling around Claude Code, even though I agree it doesn't have this capability out of the box. I think a large part of this gulf is "it doesn't work out of the box" vs "it can be made to work with a little customization."