I'm not sure what UL would say about it but the NEC never says anything about locking something on... just that the service disconnect must be capable of being locked in the off position.
It is becoming more common to have meter/main combos...nice safety feature but a little annoying when the neighborhood vandals can go down the street shutting off power to all the houses in the dead of winter.:mad:
(yes it's REALLY my name) HVAC/ Electrical Contractor licensed Chicago,IL ... I said its OFF! Don't you trust me?