This extension is not distributed with Mercurial.
Author: Peter Arrenbrecht
Project at Google code: https://code.google.com/p/httprelay/
Runs hg serve over an http relay service for when both parties are behind firewalls. The two parties need to agree on a shared secret by whatever means they see fit. The server communicates with a relay service hosted on Google's AppEngine (appspot.com), instead of listening on a local port.
Configure your .hgrc to enable the extension by adding following lines:
[extensions] relay = /path/to/relay.py