The access proxy is for LCS. If you have a large LCS deployment you should set up an access proxy so you can do load balancing. The access proxy also allows users to connect to LCS via web. I beleive they are also required for federation.
The reason I am asking about the access proxy is because of the protcol field only allows HTTP/S, which leads me to think you need an LCS Access Proxy.
Most people here probably have large LCS deployments, and thus, already have access proxies. Guess I am gonna have to do the semi-personal thing and call TSupport. :( My LCS deployment is only 100 of 4500 users, so no access proxy....yet.