In our Missal, there are references to "where it is the custom". One, randomly chosen, such example is the use of black vestments on Good Friday.
"Custom" usually assumes usage over a considerable period. Although Canon 2 makes clear that the Code does not itself cover liturgical matters, it may be useful to consider what the 1983 Code does say about consuetudo, custom. [Furthermore, a useful discussion of the situation under the previous (1917) Code can be found in J O'Connell The Celebration of Mass (1940) Volume 1 pp 27ff..] Canon 26 states that a consuetudo vigenti iuri canonico contraria aut quae est praeter legem canonicam, vim legis obtinet tantum si legitime per annos triginta continuos et completos servata fuerit. Even if a canonical law explicitly prohibits future customs, praevalere potest consuetudo centenaria aut immemorabilis. But neither an Ordinariate considered as a whole, nor any particular Ordinariate Group, Mission, or Parish can, as such, be older than 2011. How, therefore, can "custom", in the legal sense, exist in an Ordinariate?
Only, surely, if one takes such corporate groups as being in corporate and lineal and juridical continuity with the groups which existed in the Anglican Communion before they entered into Full Communion with the See of S Peter.
It appears therefore that communities possessing such continuity are to be canonically considered as having been communitates legis saltem recipiendae capaces (Canon 25).
As far as concerns the whole Ordinariate of our Lady of Walsingham, it may be said that we came out from a community which had the following customary principle: "the minister who is to conduct the service may in his discretion make and use variations which are not of substantial importance" (C of E Canon B:5; but I am citing it not as law but as evidence of settled custom). This maxim was very generously applied, with collusion at every level.
I will add that this maxim constitutes an inherent part of the ethos and spirituality we have brought with us. Vide Dom Gregory Dix Shape of the Liturgy (1945) pp 716-718; 587-589. It has a more than centennial prescription since it goes back as far as the publication of the first edition of the English Missal in 1911; and, in a vaguer form, well beyond that. It can draw support from the writings of Joseph Cardinal Ratzinger, Pope Benedict XVI, and ... Metropolitan Hilarion!