Discovered these on the blog of Aaron Beck, one of the concept artists from Weta Workshop. Some are from the website of Leonard Barrit. He was the lead CG artist in the virtual art department.
And finally, a treasure trove of concept art from Seth Engstrom, another concept artist:
You have no idea how long that took to upload... the Seth ones are from here http://sethengstrom.blogspot.com/ I didn't do all of them, there are some drawings and so on further down.