Conso,
About 25+ years ago, a number of airlines were approached, including one that almost lost a B707 in an upset, the aircraft to be used were available and configured, and had been used by the US Navy for the same purpose --- training transport pilots in upset recovery.
The aircraft were an early model Citation, much modified and strengthened for the purpose.
We got a flat NO!! from every airline approached --- all majors ---- the answer was simple, unless it was mandated by regulation, they would not spend the money.
Tootle pip!!