Several residents in my program moonlighted using NES. It was all emergency department staffing. During my research year, I did 60 hour weekend shifts in a small resort town's 4 bed hospital. The pay was good, and included a plane tickets, because it would have been a 6 hour drive. There was medicine, peds, and surgery backup on call. Bigger trauma got loaded on a helicopter and transported to a level 2 center 150 miles away. In later years, I also did 12 hour weekend shifts at an Air Force base hospital. It was very, very lucrative; occasionally NES would call at the last minute offering double their normal rate, because someone bailed out on them.
Our program had no specific regulations prohibiting this.