In the liturgy section there are two threads (one very extensive) discussing unaccompanied singing by congregations (The New Texts: Implementation without accompaniment. Unaccompanied Singing 2).
Aside from everything else that is discussed the act of composing a single line by itself is a very good compositional discipline. I remember that both at York University and on 'Contemporary Music' courses people were encouraged to do this. Often it was their very first introduction to composition.
The reason such approaches are recommended is that it concentrates attention on just a few aspects of composition - in particular on 'line', articulation, dynamics and rhythm.
Recently I had another shot at this by writing a modal congregational Mass. Maybe it exposes my lack of originality but certainly I found it more difficult to produce music that was anything other than anodyne. I suspect this is because one is limiting one's resources even further in the following ways:
: You are tied to a text
: You are limited by the vocal range of the congregation
: A Mass setting for congregation has to be quickly learnt, so you are driven in the direction of using the same 'root' material for each part of the service. The musical material therefore has to be exceptionally 'arresting' and 'deep' to withstand this sort of treatment.
: If you use an 'equal note' Solesmes plainchant system you limit your options even further.
A way round this is to use a wider array of more sophisticated devices - changes to dynamics, articulation, rhythm etc - but this tends to make the music harder for a congregation unable to read music to learn. If you go down that avenue you are effectively asking for a 'congregational choir'. In other words you often end up striking some sort of compromise between using devices that may (or may not) make the music more 'interesting' and music that may be 'more banal' but easier to learn.
In turn this throws an interesting light on Plainchant. Many expect it to be learnt by congregations; but some Plainchant is complex and difficult and clearly designed for proffessional calibre (or very experienced) singers (i.e. monks who sing it every day!). Many people today find even 'simple' plainchant hard to learn. Is this just because it is for them an 'alien' form? Or is it because the exact nuances required for effective performance and the Solesmes monastic tradition of self abnegation create significant obstacles?
Tonality presents interesting questions:
(1) In one sense a Diatonic (Major-Minor) system of scales limits your options. After all you are 'only' dealing with two 'modes'. However if your 'line' is not so much a 'melody' as a sequence of arpeggiated chords then you are, in effect, giving yourself the option of using the language of very sophisticated harmony with the great contrasts that can be achieved from switching between major, minor, diminished and other types of 'chords'; and, of course, you can use these as 'levers' for changing key.
(2) Modes, at first glance, widen your options, because you are dealing with at least four (or eight if you include the Plagal versions) scale systems. On the other hand, if you stay in your selected mode (as I did), this tends to weaken the contrasts you get from shifting 'chords' in the way that happens with the Major-Minor system of scales. It also limits scope for melodic invention. A way out then might be to change modes; but even here I noted a certain 'sameness' between all the modes; or, to put things more charitably, you are dealing with something that is by definition rather nuanced. You do not get that sense of firm harmonic 'direction' that can be achieved with the Diatonic system. Note also that the major-minor system gives scope for nuanced (as well as more contrasting) 'chordal shifts' anyway.
Another way out is to use a greater degree of 'chanting' on the same note, relying on rhythmic patterning, dynamics and articulation to produce creative results. Again, though, this is something that can be done in the Major-Minor system of scales.
Finaly there is the question of notation. 'Modern' notation - if used to its full extent - is much more exact than neumes (whose symbols have to be learnt), let alone stemless dots. This is particularly important for defining rhythms, articulations and dynamics.