It seems like the problem that WebRTC wants to solve could be solved another way, by putting more of the discovery logic into the browser rather than the application. WebRTC wants to find peers on the local LAN, and communicate with them directly. Why not let the browser find peers, and then hand the WebRTC application a connection without exposing where that connection leads?

That said, long-term, I think networks need to stop treating non-routability alone as a firewall mechanism. Any information that this WebRTC mechanism reveals could also be exploited by any random client application, or in the case of http-based protocols, by anyone who can embed an iframe or submit a form. Consider how you'd design a network in which every device had a routable IP address, and go ahead and design it that way anyway as a defense-in-depth measure. Use encrypted and authenticated protocols even on your "private" network.