The author addresses this in the OP: "We later learned that Google’s gRPC Team was working on a gRPC-Web spec internally. Their approach was eerily similar to

The author addresses this in the OP: "We later learned that Google’s gRPC Team was working on a gRPC-Web spec internally. Their approach was eerily similar to ours, and we decided to contribute our experiences to the upstream gRPC-Web spec (currently in early access mode, still subject to change)." I suspect anyone attempting to adopt this implementation is going to hit similar issues until a release is finalized.