Do not disable last active node in pool

How would I go about creating an IRule to send traffic to a different pool? And can the pool contain the same members, just without the health checks enabled?

Something like this:

when HTTP_REQUEST {
    if { [active_members app_pool] < 1 } {
        if { not ( [active_members backup_poo] < 1 ) } {
            pool backup_pool
        } else {
             send maintenance page?
        }        
    }
}

So let's say you have two pools: app_pool and backup_pool with the same members but different monitors - the full monitors on the app_pool and a simple ping or TCP monitor on the backup_pool. If the monitor on the app_pool fails all of the members, and if the monitor on the backup_pool hasn't failed all of its members, send to the backup_pool. Otherwise there's something terribly wrong and you should probably respond with a maintenance page.