The use of Psalm 42 "Judica me" in the Preparation of the Priest can be found in different ancient liturgical books, especially of the 10th century. One example would be Cod. Sang. 339, which is kept in the library of St. Gallen Monastery in Switzerland.
Cod. Sang. 339, in many ways, reminds me of a similar Ordo Missae that was in the possession of Sigebert, Bishop of Minden [+1035], and a few others. Let us give the directions as found in Cod. Sang. 339 for Psalm 42. On page 182 we have the following directions given after the Priest has vested in the Sacristy and washed his hands:
"Post quam ecclesiam intrat dicat: 'Introibo ad Altare Dei" cum Psalmo "Iudica me Deus et discerne..." deinde Confiteor et dicat 'Converte nos Deus salutaris' 'Dignare' 'Miserere nostri Domine' 'Fiat misericordia tua...' 'Domine exaudi..' 'Aufer a nobis Domine quesumus iniquitates nostras ut ad sancta sanctorum puris mereamur mentibus introire. Per..' "
Some of the text is abbreviated for the convenience of the Priest. But, in English, the above would read as:
"After which he enters the Church, he says: 'I shall go to the Altar of God..' with Psalm "Judge me, O God, and discern..[Psalm 42], then the Confiteor and he says: 'Turn us back, O God of [our] salvation,' 'Do Thou deign,' 'Have mercy on us, O Lord,' 'Let Thy Mercy...' 'O Lord, hear..' 'Take away from us, O Lord, we beg, our iniquities that into the Holy of Holies with pure minds we may be worthy to enter. Through..."
Cod. Sang. 354 on page 8 also has similar instructions for the saying of Psalm 42 as the Priest enters the Church and before Confiteor before the Altar.
Migne's PL prints a 10th century Mass Ordo which has directions in a similar vein, although, Psalm 42 is placed AFTER "Aufer a nobis", thus:
"Et dum pervenerit ad Altare imponat per se Antiphonum: 'Introibo ad Altare Dei, ad Dominum, qui laetificat juventum mean' cum Psalmo: 'Judica me Deus et discerne causam meam de gente'"
Which is:
"And when he arrives at the Altar he imposes on himself the Antiphon: 'I shall go to the Altar of God, to the Lord, who giveth gladness to mine youth,' with the Psalm: 'Judge me, O God, and discern my cause from the nation..."
Of course, in this, we can note the influence of possible older Vetus Latina text where "Dominum" is given instead of "Deum" for the Antiphon. After Psalm 42 follow additional verses and responses, and additional prayers before the kissing of the Altar and Gospel book.
However, it can be said that Psalm 42, whether said as the Priest left the Sacristy, or while the Priest was directly before the Sanctuary, or was even entering the Sanctuary preparing to kiss the Altar, was being used.
