Uh huh, I go to choir yesterday and lo and behold we are doing Baba Yetu, as a massive choir!
On this topic too, I've contacted the composer of Grim Fandango and asked if we could use his scores. He's currently said that he might be able to send them by August! How exciting..
Does anyone know of some good small band/voice pieces from games where the scores are readily available?

You might try to contact Visa Röster. They've made two albums of a capella C64 tunes. I don't know if they've actually scored them in the way you mean though. The same goes for Press Play On Tape, although their music is mostly instrumental.

Well, PPOT have guitar tabs for a few of their songs on their homepage.
EDIT: And if you're feeling REALLY ambitious, here's a link to a bunch of Machinae Supremacy tabs. Not all of it is game music, but they've made some killer game remixes over the years, especially Giana Sisters and Sidology 1-3.