I think the "play in game as mob or npc" would be included in the duty of a GM, only more interactive with players. And btw i intended they'd controll some NMs or npcs only sometimes and randomly, not every day (the normal NMs will remain of course). I don't think it'd be so heavy in human resources.

In any case, i know that balancing this type of encounters would be not so easy (like vioon said) but they could have a try If not possibile, they always could be special npcs around (sometimes, not every day).