Tom -
There are a number of us who are looking for this.
If you visit
http://www.whmcs.com/members/dl.php?type=d&id=14 you will see the WHMCS api link for server provisioning.
How close are we - is this something perhaps a few of us could fund ? to make happen?
We are looking to stick with ProxMox however it is difficult when we need a system that will allow for automation.
We can already do this w/ a ton of other systems... but we believe in the ProxMox mission... it is difficult however for many who are getting pushed by management to do automation not to give an alternative in return :-(