I know the problem, I have a modded Xbox myself and discovered that the ftp server is kinda "awkward" to put it as gently as I can. You could write a proxy, (but I don't really see the need for it (then again, I'm just busy/lazy ;)), if you're really determined to use your regular ftp client.

I work around all this by just using `ftp` (man ftp) to transfer files, and/or use the ftp client that's on my xbox and use it the other way around (so I use the xbox as client and my PC as host).

Maybe it's even easier to write a ftp client that uses the right commands, instead of a proxy? Or maybe you could use some Win32 ftp clients (since they do seem to work) running WINE?

Anyway, if you happen to finish the proxy, I'll be more then willing to test it ;)