AFAIK there is no Rust compiler for Plan 9 or 9front. The project is using a dialect of C and its own C compiler(s). I doubt adding Rust to the mix will help. For a research OS, C is a nice clean language and the Plan 9 dialect has a some niceties not found in standard C.

If you really want Rust, check this https://github.com/r9os/r9 it is Plan 9 reimplemented in Rust (no idea about the project quality):

R9 is a reimplementation of the plan9 kernel in Rust. It is not only inspired by but in many ways derived from the original Plan 9 source code.

There isn't, though you can run it over wasm on it. I tried it a while back with a port of the w2c2 transpiler (https://github.com/euclaise/w2c9/), but something like wazero is a more obvious choice