Although the default alias we hand out to our customers is tremendously useful, and you can always just set up a 301 redirect to your website doman if you are worried about SEO isses (or make sure the webserver just answers with a 404 on the aliasdomain) we realize this may be useful for some customers to be able to do.
This feature would need to warn users that if their main domain has some problem or stops resolving properly certain functionality such as phpmyadmin may become inaccessible.