URL Redirection for Maintenance Window

So if I'm understanding you correctly, during a maintenance period you'd like WAN users to get sent to a maintenance pool, while LAN users get sent to the original pool. Correct? If so, what would cause the LB_FAILED event to fire if the original pool still had accessible members?

You would necessarily need another pool that wasn't affected by the same monitoring/intervention that you had for the VIP's configured pool, and then something like the following iRule:

when LB_FAILED {
    if { [class match [IP::client_addr] equals GDC_POOL] } {
        pool original_pool
        LB::reselect
    } else {
        pool maintenance_pool
        LB::reselect
    }
}

During the maintenance window, the original pool members , we've got 2 members and both would be made down so as per my understanding should trigger a LB_FAILED event and then the if conditional loop would come into picture.