I was just addressing (nitpicking?) your characterisation of the browser security model. Some protocols do use the HTTP proxy settings, but eg mail and news protocols don't. Granted those have been phased out of many browsers, but traditionally the browser security model was never "everything goes through HTTP proxies".
Off the top of my head I can't think why WebRTC wouldn't work with normal SOCKS proxies though, like NNTP and mail protocols did.