Is this automated? Does doing this manually do anything for Debian other than create a headache and a mountain of work?

If it were up to me, I’d consider carving off part of the Debian package namespace - like cargo—* for cargo packages, and just auto-create them (recursively) based on the package name in cargo. There are a few things to figure out - like security, and manually patched packages, and integrating with cargo itself. But it seems like it should be possible to make something like this work with minimal human intervention.