Snowflake rendezvous using Amazon SQS
Nathan Freitas suggests using Amazon Simple Queue Service for exchanging rendezvous messages. "It supports programmatic sending of messages via web service applications as a way to communicate over the Internet."
There's a command-line interface.
I'm not sure how you would send a message back to the client, and have it match up with the message the client sent initially. Maybe a separate queue per client?