RMI won't help with this.
You don't need to configure the web server, you just need to write the code that accesses those 6 machines via FTP. Thanks to the
Apache Commons Net library, that's pretty easy. The javadocs of the org.apache.commons.net.ftp.FTPClient class show the basics of an FTP client.
Mind you, that's assuming that those 6 machines each have an FTP SERVER running; just having FTP CLIENTS on them won't help.