I can't help with win 8, we don't use windows here except for win XP in a VM for test purposes.
Since you are restricted to SSH and I am not sure how full an implementation Fun_Plug uses. You may be able to use SCP http://www.openbsd.org/cgi-bin/man.cgi/OpenBSD-current/man1/scp.1?query=scp&sec=1 to do the transfer assuming that is available.
The other thing you could try, although it is not a direct copy in the NAS, is FTP. SSH will allow you to setup an FTP server on the NAS and the you could use something like FileZilla on your computer to log and do a transfer to the USB stick plugged into your computer. As I said it is not a drag and drop action but is usually much faster than using SAMBA - the protocol used for d&d.
If you reactivate telnet you could open a command shell and just use copy <fille> usba (assuming usba is the attached usb stick). When the file is copied you would then exit the command shell and logoff telnet, which is what we do here when necessary.