Hi Tim, That's a terrible situation, I hope others on the list can give you more useful advice.
Section 2(c) could be of use for future versions. You can add a note that is displayed on execution of filezilla saying the terms of use and how to view the full copy of the GPL. Section 2(c) says that modified versions must retain such a notice.
GNU GPL Section 2(c): If the modified program normally reads commands interactively when run, you must cause it, when started running for such interactive use in the most ordinary way, to print or display an announcement including an appropriate copyright notice and a notice that there is no warranty (or else, saying that you provide a warranty) and that users may redistribute the program under these conditions, and telling the user how to view a copy of this License. (Exception: if the Program itself is interactive but does not normally print such an announcement, your work based on the Program is not required to print an announcement.)
Ciaran O'Riordan