Please see
Solution 1 and my comment to it. You are right, you cannot do it. FTP is not designed the way to make it possible. Therefore, you can do it only if you use yet another service on your computer hosting the file in question.
If this is Windows OS, you can develop some Windows Service working by subscription. If a client of the service is subscribed by connection to the server, it can also pass the data on what part of file system. Internally, the service should use
FileSystemWatcher
and itself subscribe to the required events.
Please see my past answer to get an idea of such service:
Multple clients from same port Number[
^].
For development of Windows Services, please start here:
http://msdn.microsoft.com/en-us/library/d56de412%28v=vs.100%29.aspx[
^].
—SA