Okay, I got this. Well... if you can stretch the definition of "formal meeting."
The Muslim pirates who raid Rome in 846 don't just plunder the Vatican, but somehow manage to scale the city walls and sack Rome itself. They can't really hold the city, but Pope Sergius II is captured. Since the raiders are under the auspices of the Aghlabids, who pay tribute to the Abbasids, the Aghlabid emir decides to offer the enslaved Pope as part of his tribute, and Sergius manages to be taken to Baghdad before his death (which IOTL was only about six months after the raid, but hey, maybe the abundant butterflies keep him alive a little longer).
Possible but wouldn't Romans evacuate pope before? Altough it might be bit difficult if Rome is totally sieged.