Yes it is possible
A priest friend of mine wrote the Office of the Apostolic Penitentiary after the death of +John Paul II with this very question
He got an official response back ( in Latin with a Protocol number) noted that it is sufficient to pray for the Church as a general intention.
My priest friend recently published a picture of it on his Facebook site.
Here is the English translation he posted ( name removed)
1) Since prayer for the intention of the Supreme Pontiff is prescribed for gaining a plenary indulgence (Cfr. "Enchiridion of Indulgences," 4th ed., Norms n. 20 § 1), when the Apostolic See is vacant, are the faithful impeded from obtaining an indulgence?
And, insofar as they are not impeded,
2) How should this condition be fulfilled when the Apostolic See is vacant?
And God, etc.
Dated April 13, 2005
THE APOSTOLIC PENITENTIARY responds to the proposed questions:
1) No; that is, the faithful can receive a plenary indulgence even at a time when the Apostolic See is vacant.
2) When the Apostolic See is vacant, the condition of praying for the intention of the Supreme Pontiff ("Our Father" or "Hail Mary" recited once; however the faculty has been given to individual members of the faithful to recite some other prayer according to their own piety and devotion) can be fulfilled even as it would be where he alive, since the goals of his intentions, which are certainly the spiritual goods of the whole Church, persist