Well, this is how NAT works, you can't easily traverse it out and back in. So unless someone writes a patch to acomplish this i'd say no.
But to say never is a bit strong. Although I would not like my firewall doing this. For testing you should either get somone to test for you, or have a second link (dialup or whatever) to test with.
Another thing is that testing from the inside will never be the same as testing from the outside. Doing so will often give you more greif then you would like.
I know this from learning it the hard way, stuff working on the inside of our netwrok just not when crossing the border gateway, or testing stuff with DNS and using our own DNS in the process.
My wote to this kind of function would be no. It's just another "footshooting feature" in my opinion.