For anybody that ends up on this old discussion regarding the Xenica WHMCS addon. It might be interesting to know that they charge $30 per month and it seems not to have been maintained in a very long time.
For vmware there is a suite of scripts with working examples, for Xen Orchestra we have the API set, but it would still be very beneficial to get the original plan of @olivierlambert in this blog post, back on track.
I could provide a WHMCS test setup, people for testing (agents of hosting companies), and participate as much as my limited programming skills allow 