That's up to the maintainer; if they don't have any knowledge of Rust, then it's better they don't get involved anyway. They're still responsible for designing the best C interface to their subsystem as possible, which is what most of the kernel will be interacting with. It puts the burden firmly on the shoulders of the Rust advocates; who believe the task is manageable.

As for your concern about code quality, it's the exact same situation that already exists today. The maintainer is responsible for his code, not for the code that calls it. And the Rust code, is just another user.