Lost some time in this, so maybe it is useful to you: I kept getting "Bad EPRT protocol" errors trying to connect to some FTP servers with the macosx command-line client behind a NAT.

The solution is to disable the use of the EPSV / EPRT commands, using the epsv4 command. Or put it in .netrc to make it sticky.

... 230 Login successful. Remote system type is UNIX. Using binary mode to transfer files. ftp> epsv4 EPSV/EPRT on IPv4 off.

These extensions are defined in RFC 2428.