In the 1998 edition of the Lectionary for Mass, this “need” is fused with the occasion of a council or a synod. We can conclude that excluding these big events, a meeting would be fair game. Such a meeting would likely include clergy, so Sunday is unlikely. I’m thinking a convention or workshop.
The readings offered are surprisingly few: Deuteronomy 30:10-14 (the word is near), Psalm 19:8-11 (words of everlasting life, and an Ordinary Time common psalm), Philippians 2:1-4 (the preliminary to the Kenosis Canticle). The Church gives a choice of three Gospels: Matthew 18:15-20 (binding and loosing), Mark 6:30-34 (Jesus’ plans for a deserted place are overcome by the crowds), or John 14:23-29 (the Lord speaks of peace at the Last Supper).
As for music, we have:
Entrance Antiphon Cf. Matthew 18:19, 20
Thus says the Lord: Where two or three are gathered together in my name, there am I in the midst of them.
Or: Colossians 3:14-15
And over all things put on love, which is the bond of perfection. And let the peace of Christ rule in your hearts, the peace into which you were also called in one body.
Where true charity is dwelling, God is present there. By the love of Christ we have been brought together.
The entrance choices match up with the Gospel readings. I think a New Testament Canticle would be a better match for these than something from the Jewish Scriptures. But if you pressed me, I would suggest Psalm 100 or 121 or 122 or 133 or even 128.
The Communion antiphon is clearly Ubi Caritas. Settings of that are beyond counting.
The Roman Missal recommends the use of the second Holy Spirit preface. Perhaps that might add Psalm 104 into consideration for some sung text at the Mass. What would you think of that?
Some years ago, we blogged on Masses And Prayers For Various Needs And Occasions. In the GIRM, sections 368-378 cover the universal regulations on their use. You can check our brief comments here and here and here. The USCCB’s unannotated text on the matter is here.