For anyone who might be interested, this is fairly straightforward on cPanel. All you need to do is create an ’addon domain’ under the account where the MODx install is, and make sure the document root is ’public_html’ (if that’s where your MODx install is), as opposed to ’public_html/addondomain.com’, which I think cPanel has a tendency to set as standard unless you specify otherwise.
This is definitely a clean solution, but I’m currently trying to weigh up the pros and cons. If, for example, I needed to put up a non-MODx version of 1 of the sites, then I’d have to remove the addon domain and create a new cPanel account, as opposed to simply removing existing index.php etc and replacing with static site files (I think!).
Has anyone tried any of the other methods of running multiple sites from 1 install? In a way I think I would prefer separate cPanel accounts, and therefore separate FTP accounts with individual index.php files etc as per -
http://svn.modxcms.com/docs/display/revolution/Using+One+Gateway+Plugin+to+Manage+Multiple+Domains - but it sounds like you need some pretty complex .htaccess action to accomplish this.