I didn't think Will was playing at this show, but looking at this video I agree it does look a bit like him.
In an effort to find out who it is, I looked at Timothy Drury's website and found he has a link where you can send him a message. So I asked him if he played keyboards for the Eagles Millennium shows, and he replied by email that it was indeed him who played for these shows (there were 2 in Las Vegas as 'warm-ups' before the New Years Eve one in LA). So the question is answered.