Chris Taylor, an author of online unicycle riding instructions, has a page on idling that I read a couple of weeks ago. The part that particularly stuck with me was at the very end where he said, "...practice idling with your arms folded".
Well, I can't idle yet regardless of arm position, but that "arms folded" comment stuck with me as something that might be useful. During the last few rides I've been trying to fold my arms and keep my upper body as quiet as possible when riding on relatively smooth, flat sections of road. The first ride it felt about as uncontrolled and awkward as the first ride when I tried to grasp the saddle handle with my left hand. I could maybe go ten strokes at the most before letting go to regain my balance. The next ride when I tried arms folded it seemed easier to get to ten strokes and beyond. Today it wasn't very tough at all on the flats so I tried folding arms up and down gentle hills and some bumpier trails. It really isn't that big a deal as a skill in itself, but I think it has been helpful for me in focusing the balancing adjustments on subtle shifts in the midsection of the trunk of the body rather than on flailing arms. I think I ride quieter now because of the arms-folded practice.
Now I need to get back to learning to idle.