Irule to redirect all incoming traffic

Maybe you don't need an iRule. In the HTTP profile it is possible to enter a 'Fallback Host'. The help funtions notes:

"Type the fallback host to send as an HTTP 302 response, when all nodes are down."

You could also use the iRule from this SOL:

https://support.f5.com/kb/en-us/solutions/public/6000/500/sol6510.html

when LB_FAILED {
    if { [active_members [LB::server pool]] < 1 } {
         The redirect will be sent only if LB_FAILED
         was because the pool had no available members.
        HTTP::fallback "http://host.domain.com/redirect.html"
    }
}