The easiest way to achieve this would be for joe to put up a forwarding script on his own domain/sub-domain. Joe might also have the ability to set up a domain redirect so his domain/sub-domain would point to www.yoursite.com/joe/
The problem I see with this is that the solutions require that joe pay to register and in some cases host his domain, so he might not be interested in using your site anymore. Also, since you are allowed to host unlimited domians and sub-domains it might be that you’d want to actually host their domain/sub-domains for them. This, of course, requires much more advanced scripting.
In this example Joe would go out and register his domain all on his own, and then point the DNS to ns1.(2 and 3)dreamhost.com. Next he goes to your website and uses your script to add the domain into your system. Here’s the complicated part - your script will have to authenciate it self to the panel, and submit the new domain to be added - basically as if you’d filled out the form yourself and hit submit. Upon sucessful completion of that, your script would then modify the mod-rewrite rules to include this new domain.
If you’re interested, the idea of having scripts submitting things to the panel has come up before - though there’s not much informaiton aviable on it. You should be able to search and find the topic - the concept before has been to allow site visitors to set up free E-mail addresses.
Hope this helps.