The handy dandy rabbitmqadmin tool makes it simple to publish a message, but (possibly depending on the client library at the other end) if you want it to appear as json rather than a string, you need to set a few options:
./rabbitmqadmin publish exchange=foo routing_key=foo.bar.baz \
properties='{"content_type":"application/json"}' \
payload='{}' payload_encoding='string'
Look on my works, ye Mighty, and despair! 