Any of those things would be good. I was specifically thinking of just cello and flute, but other available forces are: piano, organ, one decent tenor soloist (myself), several decent treble choristers (who sound okay as a choir, but not much for soloing), and there's also a trumpet player from time to time.
I personally am a big fan of the composer Devienne, especially when you have a flautist. There are some nice flute / viola duets, which you could probably transcribe to flute / cello. http://imslp.org/wiki/6_Duos_for_Flute_and_Viola,_Op.5_(Devienne,_François) Have you asked the musicians themselves? Either of them may have / know suitable choices. I assume you are looking for prelude / postlude type chamber music and not liturgical music choices.
There are numerous works for organ and organ and violin by Latvian composers on YouTube. They tend to be lyrical, intense pieces, many of which are very expressive. Does anyone know whether they are published or otherwise available? One of the organists on several recordings was kind enough to send me scores by email, but I wish I could purchase them.
In 2007 or 2008 I directed a student ensemble consisting of a guitar, a flute and a trumpet. I transcribed Bach's Wachet Auf, ruft uns die Stimme, giving the flute the right hand, the guitar the pedals, and (unsurprisingly) the chorale melody to the trumpet.
When the trumpet player had to bow out because of a relapse of his leukemia, I wrote the Ave Maria Suite, a very simple piece in multiple movements for violin (me) flute and guitar, utilizing the chant melody. If you're interested to see it, I'll see if I can make an electronic copy available. (So far as I'm aware, it's only in hard copy.) It would be easily adaptable for cello.
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