It looks like the goal of this project is to allow Rust code to avoid having to use FFI to call C code. In order to be useful to non-Rust languages, this project would either have to expose a C-compatible interface for FFI (which somewhat defeats the point of trying to avoid C), or else the other language would need to natively support Rust FFI (which would be a large amount of ongoing labor, since Rust doesn't have a stable ABI).

I believe that it would open the door to writing a Linux-only libc in Rust, which could be cool

There is in fact already a library that does that https://github.com/redox-os/relibc