GlobalSign have a great SSL provisioning system that makes life a lot easier for both service provider and customer.
There is already an integration with WHMCS, but there also needs to be an integration with InterWorx for it to work.

information on the system:
(watch the movie, it does a great job of explaining how awesome it is).

Open Source php lib:
in order to make the plugin.

This shouldn't be a too advanced plugin to make it work.
The system have support to automatically assign a dedicated IP to the account before provisioning the SSL. And since the system doesn't use DNS cache, the IP change doesn't delay the provisioning. The IP function is optional, but would be great to have as well.