Here's something from the infamous Father Scott of the SSPX, written upon the defection of a Father Vanderputten from the Society:
He himself has obtained a celebret from Cardinal Ratzinger, and now celebrates the Indult Mass, under the condition that he places himself under a diocesan bishop within six months.
Again, I would find it odd that the Vatican would approve of a "free lance" priest celebrating the Tridentine Mass outside the jurisdiction of a bishop. Every priest has to be attached to a bishop or superior.
A few hits in Google for: celebret Tridentine "Ecclesia Dei"