The question about adding Add-On Key

Rather than check pool member status in the rule, I would use priority member LB to handle the load-balancing part. For example:

pool www_pool {
   min active members 1
   member 10.1.1.1:http priority 100
   member 10.1.1.2:http priority 90
   member 10.1.1.3:http priority 80
   member 10.1.1.4:http priority 70
   member 10.1.1.5:http priority 60
}

This ensures that server1 will be picked if it's up. If it's down, server2 will be picked. If server2 is down, server3 will be picked, and so on.

As for the URL rewriting, you should be able to do that in an LB_SELECTED rule. You can use the "LB::server addr" command to determine which server was picked and the "HTTP::uri" command to rewrite the URL. Good luck!