Hi
Don't you think that the webmail access URL mail.<customer domain> is better to redirect or proxy to webmail site hosted on the control panel server?
redirect webmail access address mail.<customer domain>
The reason the separate webmail exists per mail server is to give better performance. The central webmail is convenient but it has to connect to the mail server where your customers' messages are actually stored. This is fast if your servers are local to each other but if you have servers in multiple continents, as many Enhance customers do, the performance is worse than a webmail which is running locally to the web server. That's why we offer both options.
a year later
All mail.customerdomain webmail instances point to the same installation of Roundcube, running under the use roundcubelocal. If you su to that user you can add a redirect to the top of index.php or in a .htaccess file.