this looks really sweet. i've never understood why gRPC limits itself to protobufs only when the protobufs have a canonical json representation. i'm glad that twirp is fixing that piece.
On GCP, Cloud Endpoints proxies will transparently translate back and forth between protobufs and the canonical JSON, allowing either representation to be used. So if you're on GCP and don't care about vendor lock-in, that's a solution.
There is also the grpc-gateway project for JSON transcoding https://github.com/grpc-ecosystem/grpc-gateway