Funnily enough, McKenzie was in the original LOTR trilogy, briefly, playing one of the Rivendell Elfs.
I guess it's possible it's the same character, an Elf called Lindir. A quick google search on the character shows he made one appearance in The Lord of the Rings (on page 237 according to lotr.wikia) as someone who listened to a story Bilbo was telling at Rivendell.
So, unless his casting is just a bit of continuity from LOTR to The Hobbit we can maybe look forward to hearing McKenzie actually speak more than two lines. Now they just need to make Jemaine Clement a Hobbit or River Man or something and we Conchords fans can be totally satiated!