While looking for the various Office and Masses in honour of St James conceded to Spain (the feast of the Translation of his body to Compostela, on the 30th of December; the feast of his Apparition as Santiago Matamoros on the 23rd of May; the Vigil, on the 24th of July, of his feast; and his feast itself, on the 25th of July, with an Octave ending on the 1st of August) – as I am planning to walk the Camino in four years' time, and am beginning to make remote preparations, spiritual and temporal – I made a most amazing discovery: amongst those Masses for use in the Spanish realms, in a special Votive Mass for pregnant women and those undergoing the pangs of childbirth, the Collect, Secret and Postcommunion are addressed to the Father, the Son and the Holy Spirit respectively.
Now, there are plenty of orations addressed to God the Son (though of course most are addressed to God the Father), but this was the first I have ever come across, appointed for use in a Roman Rite liturgical book, addressed to God the Holy Ghost. Fellow students of the sacred liturgy, this is a belated Christmas gift to you.
Years ago, I read Jungmann's The Place of Christ in Liturgical Prayer, which confirmed that liturgical prayers, apart from some in the Armenian Rite, are hardly ever addressed to God the Holy Spirit. Now, the Third Person of the Trinity is certainly invoked in hymns and antiphons (such as the Veni Creator Spiritus and the Veni Sancte Spiritus of the Roman West, or the troparion O heavenly King of the Byzantine East), but prayers in the strict sense are nearly always addressed to either the First or, less often, the Second Person of the eternal Triune Godhead.
The Postcommunion that I came across is found in a book available online, entitled Missæ quæ in Hispania specialiter celebrantur, pro celebrantium commoditate congestæ (Barcinone [Barcelona], 1833), on page 83, where it is part of Alia Missa ad honorem gloriosæ Virginis pro Mulieribus prægnantibus, aut alias in partu laborantibus (Another [Votive] Mass in honour of the glorious Virgin for pregnant women, or for other women labouring in childbirth):
Beatissime Spiritus, per quem condita sunt universa: suppliciter tuas exaudire digneris famulas, pro quibus has tuæ porrigimus pietati preces: ut quemadmodum tuo felici, et sancto juvamine optant liberari; ita suffragiis beatissimæ Matris Dei, lumen gratiæ tuæ, in earum celeri expeditione infundere digneris. Qui cum Deo Patre, et eodem Filio vivis, et regnas Deus per omnia sæcula sæculorum.
(Most blessed Spirit, through whom all was created: deign graciously to hear thy handmaids, for whom we suppliantly offer these prayers to thy piety: that in whatsoever way they hope to be delivered by thy happy and holy aid, even so, at the suffrages of the most blessed Mother of God, deign to pour forth with speedy expedition the light of thy grace into them. Who with God the Father, and the same Son, livest and reignest God, through all ages of ages.)
This prayer is rather too prolix and complex compared to the classic prayers of the Roman liturgy, but it is still most worthy of note.
In particular, I had often wondered (in that artless way of mine) as to how the closing doxology of a prayer to the Holy Ghost would be phrased – would it end "In whose unity the Father and the Son live and reign, God, world without end", or how? In the event, it appears that the ending used is all but identical to that used for prayers ad Filium, simply adding et (eodem) Filio and removing in unitate Spiritus Sancti.
In any case, I had best reiterate my long-term plan to walk from Roncesvalles to Santiago de Compostela in 2019 (hopefully, during Eastertide). Why then? Because I ought have amassed enough leave from work by then, and when three friends of mine walked that same pilgrim path in 1997, they set off in Easter Week and arrived in time for Pentecost, which seems a good plan to me also.