I like the way they do it at the Marriott Ko Olina (we own there and go at least every other year). They come by, and if your stuff is on a chair without you, they put a big sticker with the time they came by on it. If after an hour, it's still there, they remove your things and it goes to the towel shack. That way, they have proof how long it was sitting there.
Still, at the Marriott, we rarely have problems finding a chair in the afternoon. It's much less crowded to me (I've walked around Aulani, and I can see how many more chairs are so close together).
I don't think it's necessary to go down and hold multiple chairs. We only need one or two for the four of us. I like to read in the shade and let the kids swim, so they don't need a chair for much time!