Boeing did it in 1981, with a modified B737-200. They had a cockpit in the cabin for back-up.
I believe it flew automatically across the Atlantic, the pilots taking over for the landing.
It was 30+ years ago. I think the aircraft was painted in NASA colours.
It stayed in the same time box. If it fell behind, the aircraft would speed up for a short time. If it became early the aircraft would slow a little. All so the B737, stayed in that time box.
It was successful, yet it was during 1981 !