Have you by chance seen this FAQ?
http://www.iea-software.com/emerald/faqs/proxy-incoming.html
It should help for what you want to do.  A couple of side notes:
* You should NOT have server based proxy configured.  In your servers
  config, all should hsould none for auto proxy.
* You just need to list the megapop servers in your servers list in
  the Emerald admin with the correct secrets.
* With RadiusNT 2.5.212 and higher, you do NOT need to setup the 
  treat as local/radroamdomain dummy entry.  Just make sure your
trimnames
  flag is set to 3 and it will handle it automatically for you.
--Dale E. Reed Jr. Emerald and RadiusNT__________________________________________IEA Software, Inc. www.iea-software.com
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