Wow, down votes for a question. As a non rust developer, and non BSD user, but tangently aware of these things I am just trying to understand the energy of these communities. Guess I got my answer.
OpenBSD is a tier 3 platform for Rust because of their release policy (frequent releases, no guaranteed backwards compatibility). Unless that changes it's unlikely that you'll see OpenBSD promoted beyond tier 3. From the OpenBSD side it's unlikely they'd adopt a language that's relegated them to the lowest priority.