Auto last hop in Virtual Server Config

From the support site:

Auto Last Hop is a setting that allows the BIG-IP system to track the source MAC address of incoming connections and return traffic from pools to the source MAC address, regardless of the routing table.

See: http://support.f5.com/kb/en-us/solutions/public/13000/800/sol13876.html?sr=33722390

So this has to do with return routing through an upstream device. To do what you're asking for, your best bet is probably an iRule. Here's an example.

when CLIENT_ACCEPTED {
    set default_pool [LB::server pool]
}
when HTTP_REQUEST {
    if { [active_members $default_pool] < 1 } {
        pool sorry_server_pool         
    }
}