As far as i can tell there isn't any way in the Spacewalk UI to view a list of registered Proxy servers or to show a list of systems using a particular Proxy server. Going to the system page for a Proxy server shows no indication that it is one. I don't have an RHN Satellite install to hand, but in RHN Hosted (which i know is different) the list is available as https://rhn.redhat.com/rhn/systems/ProxyList.do and the systems using Proxy as https://rhn.redhat.com/network/systems/details/proxy-clients.pxt?sid=number
There should be a list on the right hand side under the "System" item called proxies. Systems (on the top) -> Systems (on the Left) -> Proxy This should list the proxies for an org. You can see if a particular system is connecting through the proxy by going to a system and clicking on the "Connection" subtab. You can see the list of systems using a proxy by going to the proxy server (Systems -> click on the proxy's name) -> proxy -> systems using proxy. Does this satisfy your request? (Sorry for the delay).
Mass-moving to space13.
We have needinfo pending too long, closing with INSUFFICIENT_DATA.