I wonder why they didn't immediately FFI it: C is the easiest lang to write rust binding for. It can get tedious if using many parts of a large API, but otherwise is straightforward.

I write most of my applications and libraries in Rust, and lament that most of the libraries I wish I would FFI are in C++ or Python, which are more difficult.

Protobuf sounds like the wrong tool. It has applications for wire serialization and similar, but is still kind of a mess there. I would not apply it to something that stays in memory.