@Ignoto, sorry for the two-days’ delay in responding to your important question concerning choirs singing verses of responsorial psalms or adding harmony and/or descants to psalm responses sung by the congregation.

My response is that, as far as I know, the GIRM and other legislative documents dealing with the liturgy contain no norm concerning either of these practices, but a legitimate custom apart from (praeter) the law, or perhaps even contrary (contra) to it, has been established over the course of the 45 years since the Lectionary for Mass was promulgated in 1969. And this custom has been established at least in some parishes in perhaps most dioceses of this country.

The 1983 Code of Canon Law contains the laws on custom (consuetudo) in its canons 23-28:

Canon 23 A custom introduced by a community of the faithful has the force of law only if it has been approved by the legislator, in accordance with the following Canons.

Canon 24 §1 No custom which is contrary to divine law can acquire the force of law.
§2 A custom which is contrary to or apart from Canon law, cannot acquire the force of law unless it is reasonable; a custom which is expressly reprobated in the law is not reasonable.

Canon 25 No custom acquires the force of law unless it has been observed, with the intention of introducing a law, by a community capable at least of receiving a law.

Canon 26 Unless it has been specifically approved by the competent legislator, a custom which is contrary to the Canon law currently in force, or is apart from the Canon law, acquires the force of law only when it has been lawfully observed for a period of thirty continuous and complete years. Only a centennial or immemorial custom can prevail over a canonical law which carries a clause forbidding future customs.

Canon 27 Custom is the best interpreter of laws.

Canon 28 Without prejudice to the provisions of Canon 5, a custom, whether contrary to or apart from the law, is revoked by a contrary custom or law. But unless the law makes express mention of them, it does not revoke centennial or immemorial customs, nor does a universal law revoke particular customs.

The “legislator” in these canons would be the diocesan bishop, not the pastor of a parish. And that legislator would not be the supreme legislator (the pope) since the latter is the one who gave us the written law (lex). The legislator either approves the custom which is apart from or contrary to the law, or he does nothing and the practice acquires the force of law after its being observed for a period of 30 continuous and complete years.

After the promulgation of the 1969 Lectionary for Mass, there was a flurry of compositional activity preparing musical settings of the responsorial psalms contained therein. That activity continues to this day. Not all settings were/are just for congregation (the reponses) and psalmist (the verses). Some also involve embellishments by the choir which go beyond their role as members of the congregation and only singing the responses in unison.

In the 10 1/2 years that I directed choirs between 1969 and 1981, the choir would on occasion sing the verses of a responsorial psalm or add harmony/descants on the response. Usually this would occur at a more solemn celebration. It may have been my own composition, or a fauxbourdon setting of verses, or a harmonized Gelineau psalm tone, or one of the through-composed responsorial psalms published by The Composers’ Forum, or one of Alexander Peloquin’s Songs of Israel. Were I leading a choir these days, I certainly would use on occasion the fine responsorial psalms of Fr. Jim Chepponis (several of which are included in Worship IV).

To be fair, I should add that not all interpreters of canon law would agree that these practices of choirs are legitimate customs. I remember that in the mid-1980’s some US bishops expressed the canonical opinion that all liturgical laws were part of the constitutive law of the Church. Accordingly a diocesan bishop would be unable to dispense from any of these laws. As a corollary to that, I suppose they would also have held that customs contrary to the law would be unreasonable and, therefore, prohibited. However, I am unaware of any canonical experts who gave/give that interpretation. (I am also unaware of any diocesan bishop making a statement or promulgating a particular law for his diocese which expressly prohibited a choir’s greater involvement in the singing of responsorial psalms.) Because our rites have been reformed subsequent to VII and things once prohibited are now permitted – even some practices which do not conform to general principles of the reform – it seems to be a stretch to argue that all liturgical laws are constitutive law.

It depends on how "community" in canons 23 and 25 is interpreted. My commentaries on the 1983 Code are in boxes in my garage. Perhaps if someone has access to the CLSA commentary or another, they can shed some light on your question. Otherwise, I'll start going through my boxes later this afternoon.

IIRC, a parish is not a juridical entity that can establish a custom with the force of law behind it. It has to be a diocese or prelature or religous order (perhaps certain abbeys that have a higher juridical status as well), et cet.