Would you consider adding a feature in the next version of FTP2 to accomplish this. This is a log from CuteFTP. Cute is automatically substituting the PASV address with the already known host IP address.
COMMAND:> [6/26/2012 10:43:10 AM] PASV [6/26/2012 10:43:10 AM] 227 Entering Passive Mode (10,10,1,2,195,151) STATUS:> [6/26/2012 10:43:10 AM] Substituting received PASV address 10.10.1.2 to server address 72.44.xxx.xxx.
The feature is already there. The property name is PassiveUseHostAddr.