Is it possible to attach an irule to an HTTPS virtual server ?

Yes. Leave your HTTP to HTTPS redirect iRule on your port 80 virtual server. Then this can be your iRule for your 443 virtual server:

when RULE_INIT {
  set static::maint_page "Sorry! This site is down for maintenance."
}
when HTTP_REQUEST {
  set maint_prefix "/maintenance.html"

  if { [HTTP::uri] equals "$maint_prefix" } {
    HTTP::respond 200 content $static::maint_page "Content-Type" "text/html" "Connection" "Close"
    return
  }

  if { [active_members [LB::server pool]] < 1 } {
    HTTP::redirect $maint_prefix
  }
}