411A
I would have thought that too until I visited, in my current job, a US operator of a BBJ and two DA 900s without an ops manual and did not have any duty or flight time limitations. "We do what the Boss asks" was the answer from the Chief Pilot, who is furloughed from a FAR 121 carrier, btw.
My current manual has well-thought limitations, waivers are only given in special cases, with concurrence of the Chief Pilot and either the Safety Officer or Standards Captain after the PIC has the crew's agreement on the waiver request.
GF