Maybe seting up a Firewalling system and making the FTP daemon only accept connections from certain IP's (This would work well if all owners had static's) Maybe change the port number of the FTP server every month or something would help too. (Make sure you have portsentry or something on the server to detect portscaners!)
Urizen
***EDITED:
There's the ticket! Setup a Firewall with a VPN to access the FTP server, and give owners the VPN clients and the Private Key. NetScreen has a nice off the shelf product to do this if you don't want to brave it manualy (
www.netscreen.com I think) it offers 2 layer 1024bit 3DES encryption and MD5 password shadowing etc etc.. very nice product. I just installed one at work.