CPanel doesn't support it at all, it's clearly stated in their faq. About plesk, i really dont know their support behind nats, but seems they dont support either.
From experience I know that Cube Panel (http://www.cubepanel.com) works nicely behind a NAT.
Another panel that I know that could support that setup is ispConfig and is possible that other panels also do. There are many posts about control panels, and I think thery are all named here, it may be worth having a look through their websites or emailing their sales teams asking the same question.