Geoff Colmer has written and made available a descant for 'O Come All Ye Faithful'. It is available to download free from sibeliusmusic.com

Geoff writes:
This is a baroque style cornet part (played by the principal or soprano cornet) when playing the SA Green Carol Book or Red Hymn book carol, "O Come All Ye Faithful" in Ab (concert pitch) for Brass Band. In both books it's a simple 4-part harmony and this part should be played on the final verse. Should work OK. Yes, it's that time of year!
Should go some of the way to help prevent boredom setting in when carolling!