Admittedly, it’s rare that any Pope celebrates in other than the Roman Rite but it is theoretically possible. He is technically the one person who is omni-ritual, meaning that he may celebrate in any Rite he so chooses, freely and at his own discretion. For example, I believe PP Pius XI once publicly celebrated according to the Byzantine Rite (Italo-Greek usage), and John-Paul, IIRC, publicly celebrated according to the Byzantine Rite (Ukrainian usage) at least twice. The same applies to the few surviving non-Roman Western Rites as well.
