Funny how life works sometimes. I did a hack to my camera's AF assist light to avoid buying an SC-29. But then shortly after that I saw a great deal on an SC-29 on ebay, so I got one (will blog about it soon).
Meanwhile, I was experimenting with the AF assist and I observed what I thought was a fluke. Nikon's AF Assist works when the autofocus mode is either on Auto (the camera chooses the focus point), or when the center AF point is selected. If a different AF point is selected, the camera's AF Assist doesn't work.
I had assumed that the same was true for the SB-800, and all other accessories with an AF Assist light (the SB-900, SB-600, SU-800, SC-29, etc.). Apparently, it's not true. With these other accessories, the AF Assist light can work even when the AF point is not selected, depending on the focal length selected. In fact, this is documented in the SB-900 and SB-800 manual (but not the SC-29 manual):