Found one!

Here's the prayer during the imposition of the pallium, according to a CBS news story:

Blessed be God, who has chosen you as shepherd of the universal church, entrusting you with this apostolic ministry. May you shine brilliantly during long years of earthly life, until, when called by our Lord, you are vested with immortality as you enter his celestial kingdom. Amen.

I think there was more to it than this, but I can't remember any of it.