Any proxy server should do the trick, right? The trick will be having the new third party proxy server validate user credentials against the same user repository as your RI server.
You could use LDAP to do this if you configured your RI server to use an external LDAP user repository and if your new proxy server thingie did the same.
Otherwise you would have to define your users in multiple places.
Mark
#webMethods#webMethods-Architecture#webMethods-General#Integration-Server-and-ESB