Proxy based on Multiple Realms

What I want to do is proxy requests based on being in multiple realms. For example: Realm1/username.Realm2 Where is Realm1 is "host" AND Realm2 is "some.fqdn" then proxy to xxx.xxx.xxx.xxx Specifically what I am doing is trying to use FreeRadius to proxy for AD Domains. I want to enable host based authentication (i.e. host/ workstation.domain.name ) but for multiple domains. I believe proxying is the only why to accomplish this. All I can find are references/warnings to making sure that I DON'T do this by mistake. Problem is I believe this is what I must do. Is this even possible with FreeRadius? Thanks Bob