envoy (the proxy) can transcode RESTful APIs to internal grpc services and vice versa. you can even map like url params etc. to proto fields. it works well, even server streaming.
I was pushing at one point for us to have some code in our protobuf parsers that would essentially allow reading messages in either JSON or binary format, though to be fair there's some overhead that way by doing some kind of try/catch, but, for some use cases I think it's worth it...