Best practice for unavailable pool

Why do you think it requires one iRule per pool? This does assume that the correct pool has been assigned by this point in time. You could also do a check of the nubmer of active members in the LB_SELECTED event.

when HTTP_REQUEST {
if {[active_members [LB::server pool]] == 0} {
    set MaintenceMessage "We are out for maintenace
    HTTP::respond 200 content $MaintenaceMessage ; return
    }
}