Well...
#1
you could enable root login for ftp in the proftpd.conf but that is not exactly recommended.
#2
you could ssh to the server cd to the dir and wget it from somewhere or create the index.html from there.
#3
what I recommend. Using the control panel.
A domain can be set as default for an IP on the Server > IP Addresses page. Click on the number in the "Hosting" column on this page opposite of needed IP address. Then choose the domain you wish to be default one. And push "Set As Default" link.
So when the ip address is requested the client is sent to the default domain chosen for that ip...