I did see some interesting things on this NOAA page under the Technical Definitions and Computational Details heading, but I'm sure you've read that already.
The answer to the SO question "Position of the sun given time of day, and lat/long" and the above may actually be all you need.
As a side note (it doesn't answer your question directly), is there a reason you can't pull the NOAA data and use it as a lookup table instead of calculating it? Storage tends to be relatively cheap these days.
