Unfortunately the set of items that are re-exported is not available to Cargo. When PubPriv Dependency implemented and is widely used, then it is definitely worth trying. In the meantime, I don’t know if RustDoc can make this happen, but I suspect it is looking at the tree in the wrong order.
Another approach that has been discussed is making the docs.rs build setup into a widely useable cargo subcommand. docs.rs needs to build the docs for the new library but for all dependencies link to the already built and hosted version.