I suspect the pre-flight chat would achieve a level of attention similar to that accompanying the usual FA's routine on commercial flights and thus become another box-ticking exercise.
My experience has been better than this. I give "passenger" briefings to non airplane people who are carried as working crew on science/research flights in the DC-3's. Those flights are conducted under a restricted authority, as the airplane cannot be made to fully comply with the design requirements (cabin flammability). Unlike the FA standing at the front of the cabin with a canned presentation, I'm looking at each person as I give the briefing, and assessing their understanding. Sometimes the briefing will include the availability and use of immersion suits, for low level oceanic flying. These briefings will be followed by at least "Is that all understood?", for which I will not accept less than a spoken response from each person. This will have been a ten to fifteen minute discussion with demonstration, not a quick video, or waving of FA arms. There is no doubt about the purpose of the briefing, and no rush. Any points which are not clear, are reviewed, until they are. Each of the people wants to fly on the aircraft for their own science reason, but their presence requires their greater than normal familiarity with the emergency drills for the plane. They are willing.
Of course, I can do this, as the group of people will be six to ten, not a cabin of a hundred plus, with ear buds in. So similarly, a group of people paying the big bucks for a vintage ride, have a greater than pedestrian interest in the goings on of the plane, and once briefed, the emergency procedures. These people don't rise to the level of "crew", but their involvement exceeds that of "just going to visit grandma a few hundred miles away".
This is where the regulator has a role to define, review, and approve a more detailed than normal passenger briefing. The only reason that the briefing that I, and/or the flight crew of the day, give for the science flying on the DC-3's does not require "approval", is that the regulator knows it's being done correctly to begin with.
For the B-`7 accident, I cannot say if a better passenger briefing would have improved the outcome, I'm sure that the final report will present some findings. But, it would be helpful if the operators of these flights treated each flight more like a "crew mission" rather than a spin around the patch with passengers.