Supporting custom (private) registries in sdist packages

[BatmanAoD]

When using a custom private registry, what is the most "correct" way to configure registry support for consumers of an sdist?

In my team's Rust repositories, we simply add a .cargo/config.toml to the workspace root of any workspace with crates that will be published to, or consumed from, our private registry. This is fine for Rust projects consuming Rust packages, but for maturin sdist packages, what are our options?

Consider a Rust workspace where the bulk of the code is in crates/lib, and a Maturin package for Python consumers is defined in crates/python. It appears that, by default, the crates/python/.cargo/config.toml file is included in the sdist, but it doesn't appear that the file is actually used by pip install, because even if I define the private registry in that config file, I get this error when trying to install an sdist generated by maturin:

        registry index was not found in any configuration: `<custom>`
      💥 maturin failed
        Caused by: Cargo metadata failed. Does your crate compile with `cargo build`?
        Caused by: `cargo metadata` exited with an error:
      Error running maturin: Command '['maturin', 'pep517', 'write-dist-info', '--metadata-directory', '/tmp/pip-modern-metadata-r2862ymr', '--interpreter', '/root/.pyenv/versions/3.9.20/bin/python3.9']' returned non-zero exit status 1.

I can get it to build by putting the Registry definition in my home directory's .cargo/config.toml, but is this the only, or the best, option? Is there a way to bundle the registry metadata (index and credential-provider) in the sdist itself somehow?