Defining clients in virtual servers

Is there any way to get one virtual server or the default server to proxy requests to a virtual server using proxy-to-realm without having the client configured globally in clients.conf or equivalent, but only in the virtual server. Or have the client defined globally but have the radius key overridden by the client specified in the virtual server config. So I have one virtual server listening on a specific IP and port, which looks at the nas ID and proxies to a realm correlated with that nas id. Essentially what I'm trying to achieve is to have clients with a dynamic (unknown) IP address, but different radius keys depending on the realm identified in the nas id.