Due to a lack of enforcement cases to establish legal precedence, whether or not a flight is engaged in training operations under the rule would likely be determined on a case by case basis. However, the FAA Chief Counsel wrote an opinion on 5-6-81 for an operator and that opinion states 'if a tow plane is used to tow a glider to altitude for release then returns to the same airport as he departed, that tow plane is engaged in a "training operation" within the meaning of the reg' and not required to have an ELT. So having an authorized instructor on board is not a silver bullet qualifier.
More trivia regarding Hale Boggs, one of pax in the Alaska 310 disappearance. Hale Boggs' seat was assumed by his wife Lindy Boggs who went on to become a noted politician in her own right. A bridge crossing the Mississippi River was built and named after Hale Boggs in 1983. It's a suspended deck, cable stay design, first bridge of this type to be constructed in the US. The bridge was being built to replace a ferry boat named the MV "George Prince" which provided a river crossing between Luling/Destrehan, La. Ironically, in 1976 the ferry was struck and sunk by a Norwegian tanker while the bridge was under construction. It was the worst ferry accident in the US resulting in the loss of 78 lives and triggered numerous maritime reg changes. Since the ferry was not operating "for hire" it was exempted from a number of safety regs (go figure). Hale and Lindy Boggs youngest daughter became a Washington insider reporter, journalist and author, she is often seen on the Sunday morning TV show "This Week With George Stephanopolis" as a regular. Her name is Cokie Roberts.