Creating a webRTC peer *without* a browser, with just a JavaScript interpreter
Very late answer, but I think it's good to re-evaluate this question, because a lot has changed since this question was asked.
I assume this question was asked because there was no native support for webrtc yet at the time. But there is now. Android, iOS, Windows, Linux and OSX all support native webrtc libraries now.
The native libraries can be used to create a peerconnection and setup a stream to another client (cross-platform). If you want to create any webrtc-based client application without using a browser, the native libraries are the way to go. No silly standalone javascript engine necessary.
I think you could use a node.js server to do so. There's a npm package bringing webrtc capabilites to nodejs : node-webrtc.
The best way to do this right now is to create a node-webkit application. The unified node + browser context gives you the best of all worlds.