From http://technet.microsoft.com/en-us/library/hh849703.aspx the supported values are IEConfig, WinHttpConfig, AutoDetect, NoProxyServer and None. The default value is None.
WinHTTPConfig can be set using netsh http://technet.microsoft.com/en-us/library/cc731131%28v=ws.10%29.aspx
AutoDetect will use the wpad protocol to determine proxy settings http://en.wikipedia.org/wiki/Web_Proxy_Autodiscovery_Protocol which can be kludged with a wpad file
Good Luck!