That's against canon law. Cardinal Hoyos says in his letter to the General Chapter:
On the other hand, it is clear that no priest is obliged to make use of this right. In this way, an atmosphere of freedom and trust can arise in this area, which stands in opposition to every exclusivity and every liturgical extremism. The "Fraternity of St. Peter", as its name already says, can only be a family of brethren, who mutually accept each other with fraternal love, and who are united wholly into the great family of the Roman Catholic Church, where there is a legitimate place for Catholics with a traditional sensibility, which I will defend with all my power.
This is more bull. I know the situation well. The FSSP are being strong-armed to concelebrate--"to show unity with their fellow priests" in the Novus Ordo. In other words, they are being forced to accept the Novus Ordo--which most feel is deleterious to the faith, just as the Archbishop said. Now they are caught in a bind. They felt they had an agreement with Rome. It was the reason they accepted the Pope's offer. But no sooner had they expanded and achieved a great success than Rome came down on them like a ton of bricks. They were NOT SUPPOSED TO SUCCEED!
This was precisely the sort of maneuver the Archbishop suspected might happen. He mistrusted Rome--since he knew it has never disguised its intention to kill the traditional Mass. There is no interest in the Tridentine on the part of this Pope, none whatsoever. He couldn't care less if it achieves success. He wants a different Church--one that precludes co-existence with Tradition. For a modernist--as for a traditionalist--the two versions of Catholicism are incompatible. One is ancient and traditional and believes itself to be the one true faith, the other is newly invented and has as its lodestar an ecumenical "unity in diversity." The two can never be reconciled. They are different religions.