Help, iRule connection limit, apology server.

That looks close to me. You wouldn't need the mainpool portion though, just the LB_Failed event section...like this:

rule connect_apology {
  when LB_FAILED {
    LB::reselect pool apology_pool
  }
}

This way, when the servers in mainpool max out on connections, the rule will pick up the LB_Failed event and send the traffic to the apology_pool.

Have you tested this yet? You can set the connection limits low for testing purposes and then reset them higher when you're satisfied that it's functioning as intended.

Try it out and let us know how it goes.

-Colin