Yeah, I use ocaml and protobuf. I use Piqi a lot, and it works fine. It's easy to use piqi. You write your data types in the piqi language, and then derive your .proto files from it using the piqic compiler. You can then do the normal things with protobuf compilers to generate whatever language bindings you want.

Yes, I have protobuf in two scenarios (one for binary analysis, another for custom binary instrumentation) at my work, and I find it works nicely.