There is a better way ...
On our systems, we have a domain set assign for temporary addresses and
in the DNS zone for that domain, we have a wildcard ('*') 'A' entry and modified
the HTTP template under cpanel to add a "ServerAlias %user%.ourdomain.com"
server alias to every new hosting account that is created.
Two quick modifications that only take about 10 seconds to implement ....
Once done, every account you create will automatically be instantly accessible
upon creation using a subdomain under your own branded domain name.
IE:
http://login.yourdomain.com
The user can use the subdomain immediately and continue to use it while
waiting for their own domain name to transfer and you don't have to do
any modifications to their account since their account would respond to
both their own domain name and also your subdomain.
On the administrative side for you, it also makes managing accounts easier
because you can view any user account you host by "(login).yourdomain.com"
And unlike mod_userdir, this isn't subject to any bandwidth stealing issues.