I'm sure the performance is already over (how did it go?); but I'd like to comment anyway, since I do not feel RyanK's advice is the best approach. I've accompanied the Messe Basse
on many occasions, and am also familiar with the organs at Fauré's church (La Madeleine in Paris).
use 16' manual tone to accompany singers! This makes a very muddy effect, not at all what the composer intended.
Just because the score is French does not mean that you throw on all of the reeds and mixtures! If you are playing a Toccata, that would be appropriate. But the question is about accompanying a small chorus of women's voices. The fewer the stops you can use, the better. The texture should be transparent - just enough to support the singers and give them a sense of pitch.
In French organ music most registrations will be indicated in the score - if you have trouble translating them, there are resources available on the Internet - or ask here.
I don't have a copy of the Messe Basse
here, but seem to recall that it might also be registered for harmonium (?) - if that is the case, you need to adapt the registrations for organ. The easiest thing to do is to register according to the dynamic markings.
A good rule of thumb: play mainly on the Swell, so you can control the dynamics with the expression pedal. You might have an 8' Flute/Bourdon/Gedeckt or soft string stop on the Great, coupled to the Swell - move both hands to the Great whenever you need a bit more power.
I don't use too many Great stops, but will change stops on the Swell quite often.
If your instrument has a lot of 8' stops, you should use them in different combinations (along with some judicious 4' tone) before you start adding higher pitches or reeds. This is more in line with French romantic registration practices (specifically for accompanying).
The softest sections should be on an 8' Swell string stop (such as a Viola da Gamba) plus the Swell 8' Flute or Bourdon. This is what I'd call the "default" accompanimental registration on just about any organ.
If the choir has any difficulty hearing the pitch, add a 4' stop as long as it is soft
sections you might add another 8' flue, a louder 4' stop, or even the Oboe (but I'd keep the expression box more closed than open).
For a forte
I might add an 8' Principal, or a 2' stop. A fortissimo
may use either a Mixture (if it is not too high-pitched) or a Trumpet.
The main thing is to not overpower the singers - if you cannot hear them, you are probably playing too loudly!
I sometimes use the Swell Viola and Celeste for a pianissimo
passage; you can try this alone, or with the 8' Bourdon for a different effect.