I'm acutally doing json-rpc over ZeroMQ using a scheme similar to the one described above to do file conversion tasks.

A ZeroMQ proxy using a ROUTER/DEALER pair with a bunch of REP sockets in the background.

The clients use a simple REQ socket.

All in plain old C using ZeroMQ and jansson for JSON while conforming to the json-rpc 2.0 spec.