Becarefull with this, if you use fail2ban to check for failed ftp login, you will ban the server IP or localhost IP.
This can be avoid using a regex in fail2ban.
Till, as I could see, integrating ajaxplorer with ispconfig as webftp client should not be difficult.
ISPConfig must create an ajaxplorer user/pass equal to ftpuser/pass when created (modified too when ftp pass is modified, etc), and a fixed repository must be declared in ajaxplorer.
Another option could be to use the table ftp_users for login purpose (I have to explore different options)
If this is viable, maybe I could work on it, but I don't know if you are taking the way of net2ftp as web ftp client.