PASV, FTP & Server IP Address

Is there a way to change the IP address that OS X Server sends to a ftp client during a PASV negotiation? We have a IPNetRouter that performs NAT for our network. One "public" address is dedicated to our server and all ports are simply mapped to it on that IP. This works for all protocols EXCEPT for passive FTP. During the connection, the server "tells" the client it's INTERNAL IP Address (which in this case is non-routable and the connection fails). I'd prefer it to tell the client it's PUBLIC IP Address - eventhough it may mean that local connections inside the firewall may not work. Help! :(