GTM - general approach to a single manual "switch" for declaring Disaster Recovery

In our organization, we're planning on using our GTMs to control disaster recovery - that is, we have a backup data center, which is cold, but we want to spin it up in case of a disaster, and "flip a switch" to declare a disaster, and at that point have all GTM-managed names start returning the alternate data center's IPs.

My question is, what are the general recommendations for implementation of this manual "switch"? What have people done in terms of creating a construct that is manually controlled, in order to control the GTM's logic for declaring disaster, and affecting a broad array of wide IPs, etc. managed by the GTMs? (sorry, "BIG-IP DNS")

thx!