HTTP Redirection based on pool members availability.

Well, one of the simplest things you could do might look like this:

when HTTP_REQUEST {
    if { [active_members http_pool] < 1 } {
        HTTP::redirect "http://dr-site.example.com"
    }
}

where "http_pool" is the name of the pool assigned to the VIP (along with this iRule). I'd also add that this is a good case for global server load balancing (GSLB) where users are redirected via DNS to different data centers based on health and availability.