I'm vaguely considering rewriting PAM in Rust. It's definitely something that would benefit from Rust's extra security and the code quality isn't that great. Nor is the UX. What "among other things" were you thinking of?
Implementation language is incidental. Rust is... fine, I guess? But I wouldn't mandate modules use it. What's important is to move PAM out of process and sandbox the process host (using landlock, namespaces like bwrap, whatever) so we can run sensitive code with least-privilege on both ends.