But this isn't a simple way.
Why don't you integrate a function which reads the current ip from a http site ?
http://repeater.smartftp.com
for example
Or another possibility would be to have a dns server who repeats the IP from the client.
This means
http://repeater.flashfxp.com
will point to your pc on port 80
I hope you understand this concept, but I don't know if it works and I don't know such a dns server. :-\
P.S: Sorry I hadn't seen this thread, I only posted my idea :-)