With the practical use of the Ordinariate Rite in mind, and entirely for my own guidance, I have jotted down some notes about rubrics ... how rigid they really are ... drawn from the older (pre-Conciliar) Manualists. A 1940s O'Connell is my main source. I begin with Law, its obligations, in general; then move on to Liturgical Law; and end up with a frivolously bold speculation.
LAW. We are not in conscience obliged to obey a law the authority of which is uncertain. Lex dubia non obligat; non potest lex incerta certam obligationem inducere; nemo ad aliquam legem servandam tenetur, nisi illa ut certa ei manifestetur. How are we to know whether in a particular matter a law is certain? Of the systems proposed in the old manuals, Probabilism seems to many of us the most persuasive. If in doubt between two or more moral possibilities each of which can be characterised as Probable, we may follow even that possibility which is the less or least probable, provided that it is genuinely still probable.
LITURGICAL LAW. Rubrics are either substantial, because they prescribe the form or matter of a Sacrament; or accidental when they do not prescribe form or matter. They also fall into these categories: Preceptive and directive. [There are also facultative rubrics which explicitly permit a choice; I shall not trouble with them further.] Substantial rubrics are preceptive and bind in conscience. The question to interest us is whether accidental rubrics are all, necessarily, preceptive; or whether some among them are only directive. If some are merely directive, this means that they do not in themselves bind in conscience, but simply provide the approved way of carrying out a liturgical action.
Most moral theologians, and many of the old rubrical experts, hold, with varying degrees of emphasis, that at least some rubrics are only directive. They feel that the Church does not intend that small details should oblige sub gravi. They tend not to be very generous in suggesting actual examples: it is, perhaps, easy to guess why! Even among those who incline to believe that all rubrics are preceptive, there is sometimes an inclination to feel that some wiggle-room is necessary. A distinguished example of this is Benedict XIV writing as a private theologian; having reported his own agreement with the idea that they are preceptive, he adds that one can be immune from mortal sin when breaking a rubric "propter parvitatem materiae".
CONCLUSIONS. The question whether each and every rubric binds in conscience or not, is an open question. Since the question is open, one is in conscience free to choose and follow even a less probable judgement, provided always that one has a good reason and that it still does have a degree of probability.
CUSTOM. The old writers also devote some energy to the question of custom acquiring the force of Law even when the custom is contrary to the letter of the law (contra legem). They are able to show that the SRC operated itself upon this principle. I omit a detailed discussion of this point because the necessary time has not yet have elapsed since the promulgation of the Ordinariate Ordo Missae for this to be relevant, i.e. for immemorial and unreprobated custom to have become established.
But it would be amusing to propose an argument that approved immemorial customs which accompanied these liturgical formulae when they were used, by ourselves, in the century or so before we entered into Full Communion. Ecclesiologically, this would posit a continuity within our community before and after the act of reunion, a Hermeneutic of Continuity of our very own. Similar logic would enable us to continue to apply the provision in the Canon Law of Canterbury and York that "the minister who is to conduct the service may in his discretion make and use variations which are not of substantial importance": a principle which de facto has been observed anyway within the Catholic Church by most users of the Novus Ordo for nearly half a century.