I admire Father Hunwicke's erudition but I must respectfully dissent from S Pius V: the BIG MISTAKE, THE UNIVERSAL MYTH. The new Missal is imposed, and the following is a completely ambiguous afterthought, buried somewhere in the details:
[EVERYBODY] unless approved by the Apostolic See from the very beginning of institution, or by custom, which, if that be not the very institution of those churches over the two hundred years old, is constantly to be observed in the Masses so celebrated;
This is bureaucratic speech -- as well as a fairly harsh criterion -- and the ambiguity is designed to insure that most will be completely unclear about whether the exception actually applies to them: therefore, the only safe thing to do is ....
Let's imagine something different. The question then to be put (and answered truly) is:
What is truly a mark of catholicity and of ecumenical significance? Leave out the ARCIC declarations for a moment and the patsy status of the Church of England; if you meet an old-fashioned Anglican on 1st May and you have a conversation about faith, you're already on common ground in celebrating the same feast day. This is how bridges are built! Something substantive and traditional is a predicate. Do not insist upon the latest papal fiat simply because of what Mediator Dei declares about the liturgical authority of the Roman see. That is what leads to estrangement and hatred.
First the cake. Then, the theory for cutting it.