I wonder how airliners are operated under part 91 in general too. Sometimes at work, we ferry white tails that aren't on the OPS Spec yet for Mx. Our group gets involved if they need an authorization to refer to previous operators MEL for a deferral. There is no valid MEL with FAA approval letter until it's in service and it goes on the Ops Spec. Usually, it's for some small thing like a cockpit switch annunciatior or something. I just crank out what the FAA approved policy says to, and there are flight restrictions (daytime, VFR) but I do wonder what actually defines the minimum equipment requirement. TOMATO FLAMES just doesn't even begin to cover an airplane so complex.