This is a question for the proxy admin or vendor. The Cisco Meeting Apps use standard XMPP for the connection and then media uses our TURN server. Therefore, if the proxy supports XMPP, there is no reason why this would not work.
For the WebRTC client, this is probably more of a "when will they add this support" type of question. I would ask the vendor if and when they have plans to add support for HTML5 and WebRTC. Being that the standard is not yet ratified, it may take some time.