Centralized WLAN controller backing up multiple remote sites...
I have looked around and found a blurb on the ability of a single backup controller at a main site to backup multiple remote locations (and their primary controllers)... I cannot find a direct document that describes how this can be done. I don't mean HREAP either just need a centralized backup controller for different offices and mobility groups.
Re: Centralized WLAN controller backing up multiple remote sites
What kind of document are you looking for? Cisco has many documents for 4400 controller implementation, but I don't know that they have a document that specifically discusses that particular implementation.
If you want to perform this design, ensure that you have at least T1 connectivity between sites, though more bandwidth would be better. You should ensure that capacity meets your needs, so that your single backup can support any other site that goes down (or multiple sites if you wish). Finally, you should consider simply implementing HREAP, as you mention, since that will prevent all wireless traffic from spanning your WAN link in the event that the local controller goes down.
Sorry I don't have a document to share, but feel free to ask if there's anything specific you want to know and maybe someone can help out!