Thank you for the clarifications.
I am going down the rabbit hole of integrating Rust-based components in large, complex build systems that are primarily C/C++ based. It would seem this path has been well-trod already, even though there doesn't seem to be formal acceptance of the RFC that stabilizes the rlib format.
You can also get rustc to output a list of required native dependencies as part of the build process.
My understanding from the --print docs is that flag is only available when building a staticlib. Furthermore, the native dependencies the crate uses aren't listed by native-static-libs because they are included in the .a. Is there another flag you were thinking of that would work for rlib?