(And with HKTs you can abstract this whole blog post into a fully generic one-liner applicable to basically any notion of error! I can't wait til Rust picks those up.)
Does Rust's (future) HKT implementation have an RFC or a PR available?
It does not. It's entirely speculation at this point, though many, many of us want to have it. Everything has been so focused on getting 1.0 out the door, and HKT should be backwards compatible. Special pains have been taken to consider HKT when designing APIs.