direct traffic based on information from the XML request

If you can send an example of the XML request that'd help in building an actual iRule. Otherwise,

when HTTP_REQUEST {
     some function to parse out the digits from the xml request

    switch -glob $digit {
        "*0" {
            if { [LB::status node 10.10.10.1] eq "up" } {
                node 10.10.10.1 80
            } else {
                node 10.10.10.2 80
            }
        }
        "*1" {
            if { [LB::status node 10.10.10.2] eq "up" } {
                node 10.10.10.2 80
            } else {
                node 10.10.10.3 80
            }
        }
        "*2" {
            if { [LB::status node 10.10.10.3] eq "up" } {
                node 10.10.10.3 80
            } else {
                node 10.10.10.4 80
            }
        }
        default {
             send to a default pool or node
        }
    }  
}

Of course this doesn't deal with any type of persistence and what happens if the failover node is also down.