In my hymn-playing days on an American Organ (>60 years ago) I never played in 3 or 4 sharps. As Nigel says above, you simply read the key as 4 or 3 flats and play the notes written. You have to think a bit about accidentals.

I think I came to this practice because, when I was fingering a 4 or 5 note chord, it was easier to reach all the keys, because the most frequently occurring notes are on the black keys. I don't have short or thick fingers, but it still felt easier.

And, despite equal temperament, I still feel there is a difference in the feel of the same tune between the flats and sharps keys.

One other point, leeneia - If the parts of the service are in 2, 3 & 6 flats, you don't want to tranpose just one of them to a sharp key. I don't pretend to know which keys would best relate, but 'change one, change all' preserves the composer's intention.

Phil