One quick way to figure out the WAN IP without having to setup and maintain an echoing site (i.e., repeater.flashxp.com) would be to use the local machine's traceroute.
The user could specify the location of the traceroute executable and a nearby host (i.e.,
www.myisp.com). Or even better the user could specify the whole command. You could then parse the output line by line stopping at the first non-local ip (i.e., something other than 192.168.* or 10.*).
If you really wanted to get fancy you could provide a way for the client to learn how to parse the system's traceroute output. i.e., the user would specify the traceroute executable and a nearby host, flashfxp would capture the output, separate it into space delimited cells, the user selects the field that has his/her IP address and from then on flashfxp would use that field on the first PORT command of any active ftp session. The user would only have to do this once. With a few defaults the whole thing could be done with 1 click.