From a priest reader. This is edited to preserve anonymity. My emphases:
Dear Fr. Z.: I have been transferred to a new assignment. At that time I determined to celebrate Mass ad orientem.
The bishop has received letters of complaint and through the Vicar General has asked me to turn around.
I wish to know if the Bishop has any canonical power over me on this issue?
First, thank you for trying to bring your parish to a superior way of offering worship to Almighty God.
Second, I am not a canonist. Therefore, I consulted a canonist.
Here is the slightly edited answer I received.
Canonically, it would not seem that the bishop has the right to outright "ban" ad orientem celebrations of the liturgy. Bishop Foley in Birmingham tried to do so, and was informed by the CDWDS [Congregation for Divine Worship and Discipline of the Sacraments] that his decree lacked effect. He then altered the decree to state that no ad orientem celebration of the Mass could be televised – which the CDWDS did not overturn.
So, it would seem that the bishop could make ad orientem celebrations difficult. He could, for example, mandate that at least one Mass per day be offered versus populum – [If a bishop cannot block ad orientem worship, can he really mandate versus populum? If the priest has the right to celebrated ad orientem then how can he be told he cannot? Interesting.] making it unwieldy in some parishes to reorient the sanctuary for each Mass.
It would be important to know – has this priest been assigned as pastor? If he’s been assigned as pastor, then it would be quite difficult for the bishop to remove him simply for celebrating Mass ad orientem – difficult, but not impossible. If he’s been assigned as pastor for the usual six-year term, and the bishop is nearing the end of his episcopate, and the pastor has significant support from his parishioners, then this might be a battle worth fighting (if so – urge him to keep a file with EVERYTHING in writing – if he has a phone call with the Vicar General, write up a letter to the VG immediately after hanging up saying, "What I heard in our phone call was XYZ. If my understanding is incorrect, please write to inform me of what you meant.").
In short, the Bishop can’t forbid what the universal law permits, but he can "regulate" it – (e.g., he couldn’t ban the use of the First Eucharistic Prayer, but he could say, for the First Sunday of Lent, I require all parishes to use the Second Eucharistic Prayer…). If he tries "regulating" too extensively, the CDWDS has demonstrated that they will happily step in if priests and faithful complain.
This is a pretty convincing answer.
The bishop cannot ban ad orientem worship but he might be able to regulate it to some degree. I suppose this would also extend to the language of the liturgy.