There are probably some problems with this idea, but why not just use a popular social network or similar as the lobby? For example, if there is a twitter feed "@MyWebRTCLobby" that everyone follows, clients could just @ a tweet at it with current contact details.
If random Twitter users send @mentions to "@mywebrtclobby" then no other users will likely see these tweets unless they already follow each of the various Twitter users sending the @mentions.
However if you connected @mywebRTClobby with https://GroupTweet.com you could configure things so that any @mentions (from authorized users or anyone) would be converted into actual tweets from the @mywebRTClobby account so that all followers would actually see those tweets/contact details.
