I like these shots. I've always wanted to attend a Pow-Wow...Half of my family is from Oklahoma and we have some Native American in our blood, if only a little...
I think shooting a Pow-Wow would be fun, but also challenging, like you said. You can probably only get so close to the participants, and then there's so much color, movement and people that it's difficult.
IMO, these look a little too bright. On the first one, I'd like to see some feather detail in the white feather on the left and a little more headband detail in the white area, but then you have to worry about losing detail in his black war paint.
Looks like use adjusted curves on a selected part of the second one, on the top left background where it was blown out...I'd like to see more detail in the boy's boot fur.
But I'm picking nits here. I like the photos overall. Good shots, especially at 400mm!
On second thought, I wonder if a 580EX zoomed would have any effect at 400mm? These could use some fill...