If High Mass is too short, to begin with, a vernacular hymn can be sung beforehand, during the procession of the sacred ministers and servers from the sacristy, out through its exterior door, down along beside the outside of the church and in through the great west doors (or, during inclement weather, down one of the side aisles to the back of the church), then up the central aisle to the altar. In addition to this, or as an alternative, the former custom of many churches of France could be adopted – to sing the Veni Creator, with the usual versicle and collect, before Mass. (On Sundays, of course, the usual sprinkling of holy water during the Asperges, versicles and collect would immediately precede the Introit.)
As adjuncts to preaching, vernacular hymns before and after the sermon could be sung, such singing being an old German custom. And of course the sermon itself could begin with the sign of the cross, the reading of the Scriptural text upon which the sermon is founded and developed, and a prefatory “Hail Mary”, while at its end the sermon could end with an ascription of praise, to which some suitable versicle (for example, on Good Shepherd Sunday, Jesu, Pastor bone, – Miserere nobis) could be appended, and then the sign of the cross for a conclusion.
(I pass over in respectful silence the potentially limitless extension of the duration of High Mass by the indefatigable expatiations of the preacher himself...)
Furthermore, following various customs of old time, traditional forms of intercessory prayer could be read after the sermon, such as that known in France as Prône, consisting of prayers for the living (Psalm 122, Kyrie, Pater noster, several versicles and a collect) and the dead (Psalm 129, several more versicles and another collect).
After Mass ends, at the conclusion of the Last Gospel, it is in many places the pious custom for the sacred ministers and servers to process first to the Lady Chapel and there lead the singing of the seasonal Marian anthem, with its versicle and collect. (In England, of course, the prescribed versicle and collect for the Queen would first be chanted; in France, the equivalent versicle is sung thrice before the Postcommunion.) After that, a recessional hymn in the vernacular could suitably be sung while they depart by the long way, down the central aisle, then out the west doors and back around the outside of the church to the sacristy (or, if the weather is unfavourable, returning thence via one of the side aisles).
To summarize, these are suitable optional additions to High Mass:
1. Before (Asperges and) Mass:
(a) Processional Hymn
(b) Veni Creator, versicle & collect
2. At Sermon:
(a) Hymn before Sermon
(b) Sign of the Cross, text & Hail Mary before Sermon
(c) Final ascription of praise, then versicle & Sign of the Cross after Sermon
(d) Hymn after Sermon
(e) Intercessory Prayers
3. After Mass:
(a) Marian anthem, versicle & collect
(b) Recessional Hymn
Such an addition of up to four hymns would afford a valuable opportunity for congregational singing of quality vernacular hymnody, in addition to the chanting of the Ordinary (either led by the choir, or sung in parts by them alone), and the Proper (by the choir alone). It goes without saying that the choir would typically also sing suitable Offertory and Communion Motets – and, following the French custom, it could add an Elevation Motet (O salutaris hostia, or Ave verum, or Pie Jesu at Masses for the dead).