First the disclaimer: I NEVER was able to make a A400/a800 work behind a NAT.
Some people say they did it, bit I also met some folks that did met bigfoot on the mountains.
Acording to Quintum, the EXTIP field is there to change the message on a H323 message, such a way that the other gateway/switch on the other end,also a h323 device ,will know where to send thew H323 replies to.
Aparently, the quintum replces part of the H323 message with the value on that field, and the gateway on the other end will know where to send the audio/control signals.
It should not have any effect on radius information being sent out of the gateway.
Looks like the only solution will be having the radius serer on the internal network, on the same subnet than the gateway.
But if you are dealing with a radius across the internet, looks like you may be out of luck.