I am the author of piqi-ocaml. There are no plans for suppoting grpc. I do plan however to add support for Protobuf3 (currently piqi-ocaml supports only Protobuf2) which grpc relies on, and support for some kind of stubs generation for services. This way it would be usable for grpc serialization.
Those who want full grpc support would need to implement grpc protocol in OCaml, but this is strictly outside of the current piqi-ocaml project scope.