I am currently running my ISPConfig server on IPCop's orange (dmz), and it's working like a charm!
Some mods had to be done to get there, but nothing big.
As said by hkaiser, you should get three network cards, and then reinstall IPCop.
Remember to assign different subnets to green and orange.
Now you have to assign a static internal ip to the ISPConfig box, matching the orange subnet.
When this is done, you should be able to ping the outside world from your server.
The next step is to port forward the ports you are using on your server (25,80,143, and so on) to the servers ip.
The one thing to remember is that both IPCop and ISPConfig are using port 81 as their adminport.
What I did was to change the default port on the ISPConfig server, following this thread:
Also remember to change the ip to your domains from 126.96.36.199 to 188.8.131.52 at your domain registrar.
The same goes for the dns entrys on your ISPConfig box.