I suggest that if you already have a secure tunnel back to HO from the branch you do not need to deploy these as RAPs.
Assuming you have DHCP where the APs will plug in, and that there is routing back to HO where the controller is you should be able to provision these APs normally and point them at the controllers using the LMS feild.
There are a few ways to get this done:
1. Provision the APs at head office, put them into the right group, assign the controller, and then disconnect them and move them. When they boot at the branch office they should be provisioned with the info to find the controller.
2. Using DNS. Setup your DNS server to reply to aruba-master with the IP of your controller. When you APs come up at the remove location they will check for this DNS entry, and if they find it they will try to phone home.
3. You can set a DHCP option on your DHCP server. I believe option 43 should be set to the IP of you controller, and in this way you AP can find the contoller.
Let us know if these methods work for your issue.
-ELiasz