iControl related to traffic flow and iRule applied

So, you need to use a standard Virtual Server with the required SSL profile and HTTP profile assigned and then yes, something very like what you've suggested;

when CLIENT_ACCEPTED { 

 If the Pool is up, pass it through and disable HTTP and SSL
 if { [active_members pool_name] > 0 } { 
  SSL::disable
  HTTP::disable
 } 

 If the Pool is down, do nothing and SSL will be terminated and the
 HTTP_REQUEST event will fire and display the maint. page
 else { 
  Do nothing
 }

}

when HTTP_REQUEST { ... << display the maintenance page >> }