Hi Tom,
A better solution would be to use ClearPass as the repository for these guest users. They can definitely use ClearPass API to create those guest user accounts.
As far as trying to use the controller, please refer to the ArubaOS 6.4 User Guide, page 1022, Chapter 44, External User Management. You can find that document in the documentation of support.arubanetworks.com.
Thanks,
Zach