Mariner 9
Not so daft as it sounds
Better example is when the Captain becomes incapacitated effectively leaving the FO as sole pilot!
That has happened a few times where the FO has flown an approach and landing!
Yes in that situation I presume he could log that portion of the flight as P1?
But with a type rating and unfrozen ATP it would go against other regulations in force to act as Captain/ P1 interesting what does happen in such circumstances?
Effectively he is Captain / P1 but the regulations say no
Pace