The main problem I'm running up against is that my WCF client isn't able to serialize its request to JSON properly. From the stack trace (see pastebin link below), it looks like the message is being converted to XML first, then to JSON.
This seems a bit odd since WebMessageEncoder.WriteMessage(Message, Stream) should be able to handle the JSON encoding directly if GetContentFormat(Message) returned the right WebContentFormat.
Let me know if it's worth building a real-working project for this.
Here's the relevant snippets of code (may have syntax errors):
Here's the stack trace: