mixed irule?

You can only use an HTTP:: command once the HTTP has been parsed. The HTTP hasn't been parsed in the CLIENT_CONNECTED event, so the HTTP::redirect command is invalid. You can issue a redirect in the HTTP_REQUEST event though (when LTM parses the HTTP headers in a request). If you only want to redirect the client during a maintenance window, you could use a separate rule:

when HTTP_REQUEST {
   checks to see if client_addr = any in the class
   if { not ([matchclass [IP::client_addr] equals $::private_net])} {
      HTTP::redirect http://maintenance.example.com
   }
}

Else, if you want to combine the two in the HTTP_REQUEST event, here's an example:

when HTTP_REQUEST {
    checks to see if client_addr = any in the class
   if { [matchclass [IP::client_addr] equals $::private_net]} {
      snat automap
      use pool g_pool
   } else {
      HTTP::redirect http://maintenance.example.com
       snat none
       use pool g_pool
   }
}

Aaron