IRULE REDIRECT ANOTHER URL

when LB_FAILED {
    HTTP::respond 302 Location "http://cloudsite.com/sorry_page.html" Retry-After 600
}

OK then I would use a single pool with priority activation for the cloud member. In the example below, 172.18.72.70/72 are your local pool members, while 144.136.100.22 is the IP of your cloud site, and this pool member will be used only when both your local pool members are down.

pool pl_mysite.com_80 {
    min active members 1
    slow ramp time 0
    monitor all http
    members {
        172.18.72.70:80 {priority 2}
        172.18.72.72:80 {priority 2}
        144.136.100.22:80 {priority 1}
    }

You'll need access from your F5 to the cloudsite IP on port 80, and if you don't have it already enabled, may need snat automap enabled on the virtual so that traffic to the cloud site returns via the F5.