Under the definition of an experimental aircraft, air racing is listed, 21.191. Deep down this was still a Mustang. The only real change of note was the elimination of the scoop for the boil off system. Structurally the Mustang is painfully simple and strong. Yes it had clipped wings and the elevators tips were shortened but its basically a wing, engine and tail mounted to 4 longerons. That said there is no reason to expect more than the program dictated by the North American manuals 25hr, 50hr, 100hr, which would comply with 21.193 of the maintenance requirements. Obviously the racing Mustangs are a special case, they are subjected to a much more rigorous maintenance schedule and are constantly being looked at and tweaked, but that is by the crew, not a scheduled program mandated by the FAA. They also have to pass a Tech inspection by the powers that be at Reno and demonstrate both pilot ability and airframe integrity.
The precedent here is the "Voodoo incident", the other known elevator trim tab departure which resulted in Bob Hannah blacking out after a 10g acceleration, fortunately he survived and the lesson learned. Should there have been an AD? probably...but how many Mustang owners go out and run up to and over 400mph? Original Mustang trim tabs were made of bakelite, I don't know what these guys are using and it was mentioned by a crew member on another board that the Ghost's left elevator trim tab was non functional and faired into the trailing edge. Several Rudder trim tabs have been lost over the years without dire consequences which lead a couple racers to eliminate it altogether. It will come out in the end, we just have to patiently wait.
I've been following Reno since I was a little kid...I'd hate to see it end...it will be along time before the dust settles. Aside from the souls we lost there are alot of scars, both physical and mental resulting from this accident.
jim